CA1108245A - Glow discharge heating apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention provides a glow discharge heating apparatus comprising a pair of discharge electrodes in the form of hollow cylinders having one end closed, said discharge electrodes being of the substantially same shape and disposed in opposite relationship by having said one closed ends opposing each other to form a predetermined gap there-between, AC electric source means for applying an AC voltage across said discharge electrodes to cause a glow discharge therebetween, said glow discharge supplying thermal energy to the discharge electrode acting as a cathode during said glow discharge, and a liquid to be heated flowing through said discharge electrode acting as said cathode electrode to be heated with said thermal energy.

Description

BACKGR(!UI~D OF TI~E I~`~VENTION
____ ._ This invention relates to a glow discharge heating apparatus for heating a liquid through tlle utilization of a glow discharye est~blished between a pa~r of electrodes involved and is a divisional application o~ our copending Canadian Patent Application Serial No. 299,801 filed on March 28, 1978.
Japanese laid-open patent a~plication No 106252/1976 describes and claims a glow discharge heati]lg apparatus for heating a li(luid by utilizing a pllenomenon that a g:Low discharge occurring between a pair Olc cathocle and anode 0 electrodes heats the cathode electrode to an elevated tempera-ture. The glow discharge heating apparatus disclosed in the cited l atent a~plication comprises a hollow cylindrical enclosure, a tubular cathode electrode coaxially entended and sealed th~ough the enclosure, and having both ends open, a hollow cylindrical anode electrode disposed in the enclo-sure to surround the cathode electrode substantially through-our t]~e length thereof to form an annular discharge gap therebetween, a source o DC voltage connected across the cathode and anode electrodes to cause a glow discharge therebetwcen. The cathode electrode is heated with the glow discharge to directly heat a liquid flowing there-:, throug]l . ~:
I]eating apparatus of this type referred to have instantaneously heated the liquid with the simple construc-tion and still with the high efficiency. Ilowever, where high currents are required to establish the glow discharge between tlle electrodes, it has been difficult to sustain the stabilized glow discharge therebetween. T]lere have

-2-Z~5 been a fear that the glow discharge will transit to an arc discharge as the case may be. Also the electrodes have been heated to be a~ially expanded. This might result in a fear that the apparatus is broken.
Further is has been difficult to reliably control the glow discharge because of the absence of a control circuit for starting and extinguishing the glow dischargeO ~ ;
Accordingly it is an object of the present invention to eliminate the disadvantages of the prior art practice as above described by the provision oE a new and improved glow discharge heating apparatus capable of always sustain:ing a stabilized glow discharge.
It is another objec-t of the present invention to provide a new and improved glow discharge heating apparatus including means for absorbing thermal strains developed in electrodes thereby to provide a construction difficult to be broken.
It is still another object of the present invention to provide a new and improved glow discharge heating apparatus including a control circuit for easily controlling a glow discharge occurring across a pair of electrodes involved.
SUMMARY OF THE INVENTION
According to the present invention there is provided a glow discharge heating apparatus comprising a pair of discharge electrodes in the form of hollow cylinders having one end closed, said discharge electrodes being of the substantially same shape and disposed in opposite relationship by having said one closed ends opposing each other to form a predetermined gap therebetween, AC electric source means for applying an AC voltage across said discharge electrodes to cause a glow discharge therebetween, said glow discharge supplying thermal energy to the discharge electrode acting - ~ ' 2~
~ ~ , as a cathode during said glow discharge, and a l.i.quid to be heated flowing through said discharge electrode act.ing as said cathode electrode to be heated with said thermal energy.

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In a preferred embodiment of the present invention the source of voltage may comprise a source of DC voltage and a hollow anode electrode surrounds the middle portion of a hollow cathode electrode to form the predetermined discharge gap therebetween, the cathode electrode forming a flow path for the heated liquid.
. In another preferred embodiment of the present invention the source of voltage may comprise a source of AC :~
voltage and the pair of electrodes are in the form of hollow ~ .
cylinders having one end closed and substantially identical in ;~ .
shape to each other, the closed ends of the cylindrical electrodes abutting against each other to form the predetermi.ned ga~ therebetween while flow confiring means is disposed within each electrode to flow the liquid in contact relationship ~ with and along the internal surface thereof. ~ .
: In order to ensure that the glow discharge is prevented from transiting to an arc discharge, the glow discharge heating apparatus may advantageously include an auxiliary ~:
source of voltage for applying across the electrodes . . ...

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l a high voltage in excess of a discharge breakdown voltage : ¦ across the electrodes upon a discharge voltage across the - electrodes approaching a glow discharge-hold minimum . voltage~ to cause a pilot glow discharge therebetween to ~ induce the principal glow discharge.
-. BRIEF DESCRIPl`ION OF THE DRAWINGS
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The present invention will become more readily ; apparent from the following detailed description taken in : conjunction with the accompanying drawings in which:
~ 10 Figure 1 is a longitudinal sectional view of a :~ glow discharge heating apparatus constructed in accordance with the principles of the prior art, Figure 2A is a schematic sectional view of a pair of . opposite electrode useful in explaining the glow discharge;
Figure 2B is a graph illustrating a spatial voltage profile exhibited by the arrangement shown in Figure 2A;
Figure 3 is a fragmental schematic plan view illustrating how a quantity of input heat to a cathode electrode during a glow discharge is measured;
Figure 4 is a graph illustrating ~he results of .
the measurement shown in Figure 3 with the results of a :
corresponding theoretical calculation;
Flgurç 5 is a graph illustrating the relationship between a glow discharge voltage and a gap length through 2S which a glow discharge is caused;
Figurç 6 is a graph illustrating the relationship between a voltage and a current for the glow discharge;
Figure 7 is a perspective view of a modeled ion ~-. - 5 - ':
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flux usefu] in expla;ning a quantity of input heat to a :~. cathode e~ectrode resulting from a glow discharge;
Figure 8 is a graph illustrating the current-to-: voltage c1laracteristics of the glow discharge;
Figures 9A and gB are fragmental schematic plan views of a pair of opposite electrodes useful on explaining the principles of the present inventions;
Figure lOA, lOB and lOC are views similar to Figure 9A or 9B but illustrating typically electrode configurations embodying the principles of the present inventioTl;
Figure 11 is a longitudinal sectional view of one embodiment according to the glow discha~ge heating apparatus of the present inventioni ~ 15 Figure 12 is a current-to-voltage characteristic : curve for a glow discharge caused by the arrangement shown in Figure ll; ~ ~:
Figures 13 and 14 are graphs useful in explaining . the principles of the present invention;
Figure 15 is a longitudinal sectional ~iew of a modification of the arrangement shown in Figure 11;
Figures 16 an~. 17 are graphs illustrating the characteristics of the arran~ement shown in Figure 15;
Figure 18 is a longitudinal sectional view of 2~ another modification of the present invention;
. Figure l9 is a graph illustrating the characteristic of the arrangement shown in Figure 18;
Figure 20~, which appeaxs on the same sheet as Fig. 23, shows a modification of the arrangement .ll ~ z~
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l shown in Figure 18 wherein Figure 20A is a cross sectional ~r ;~ view and Figures 20B and 2~C are side elevational views of the lefthand and righthand sides respectively;
Figure 21 is a view similar to Figure 18 but illustrating still another modification of the present ~ invention;
; Figure 22 is a view similar to Figure 18 but illustrat.ing a modification of the arrangement shown in Figure 21;
Figure 23 is a view similar to Figure 18 but :~
illustrating another modification of the arrangement shown in Figure 21;
Yigure 24 is a view simila~ to Figure 18 but :~
illustrating still another modification of the arrangement shown in Figure 21; ~`
Figure 25 is a graph illustrating a leakage current calculated with the arrangement shown in Figure 24, Figure 26 -.;s a graphical representation of voltage and currcnt waveforms developed .in the arrangement of Figure 24 filled with a mixture of helium and hydrogen;
Figure 27 is graph illustrating the current-to-voltage characteristics of glow discharges occurring in the arrangement of Figure 24 filled with mixtures of helium and hydrogen having different proportions thereof;
Figure 28 is a graph illustrating the theoretical ~ relationship between a glow hold minimum voltage and ... quantity of illpUt heat to an associated electrode resulting from the glow discharge; ;

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`` ~ . ' ' ' : Figure 29 is a graph illustratlng the relat.ionship between an overlapping area for both electrodes and a : pressure of a filling gas;
Pigure 30, 31 and 32 is graphs i:llustrating how S the glow hold minimum voltage is cha.nge~ with a proportion . of mlxed gases and a discharge gap-length;
: Figure 33 is a graph illustrating the relationship between the glow hold minimum voltage and a peak discharge current;
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~: lO Figure 34 is a longitudinal sectional view of a `~
different modification of the present invention including an auxiliary electrode;
: Figures 35, 36 and 37 are fragmental perspective views of different modifications of one of the electrodes 15 ~ shown in Figure 34;
`:~ Figure 38 is a longitudinal sectional view of ~: modification of Figure 34 along with an associated electric :~
circuit; ~ :
Figure 39 is a longitudinal sectional view of another modlficat;on of the arrangement shown in Figure 34; :
Figure 40 is a view similar to Figure 38 but illustrating still another modification of the arrangement shown in Figure 34;
: ~ Figure 41 is view similar to Figure 39 but illustrat-ing a ~ifferent modification of the arrangement shown in ; Figure 34; :~
Figure 42 is a view similar to Pigure 39 but illustrating a modification of the arrangement shown in ;

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z~s . ¦ Figure 41;
: ¦ Figure 43 is a view similar to Figure 39 but . ¦ illustrating a modification of the arrangement shown in : I Figure 40;
¦ Figure 44 is a view similar to Figure 39 but ¦ illustrating another modification of the arrange~ent shown ¦ in Figure 34;
¦ Figure 45 is a view similar to Figure 39 but ¦ illustrating a modification of the arrangement shown in :
¦ Figllre 44;
¦ Figure 46 is a diagram of the fundamental used with ¦ control circuit the present invention;
¦ Figure 47 is a graph illustrating a voltage and a I current waveform developed in the arrangement shown in ¦ Figure 46;
Figure 48 is a diagram of a control circuit constructed in accordance with the principles of the present invention for driving the glow discharge heating apparatus thereof;
Figure 49 is a graph i.llustrating a voltage and a current waveform developed in the arrangement shown in Figure 48;
Figure 50 is a diagram similar to Figure 48 but illustrating a modification of the arrangement shown in .~
Figure 48; ~.
Figure 51 is a graph similar to Figure 49 but illustrating the arrangement shown in Figure 50;
Figure 52 is a diagram of another control circuit constructed in accordance with the principles of the :

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¦ present invention and suitable for use with an electrode ¦ structure including an auxiliary electrode; :~
¦ Figure 53 is a circuit diagram similar to Figure 52 . ¦ but illustrating a modification of the arrangement shown . ¦ in Figure 52;
~; I Figure 54, whIc~ appears on the same sheet as Fig. 56 .~ and 57, is a graph illustrating voltage waveforms developed : .
at various points in the arrangement shown in Figure 52;
Figures 55 through 5~ are circuit diagrams similar : .
to Figure 52 but illustrating different modifications of the arrangement shown in Figure 52;
Figure 59 is a diagram of still another control CiTCUit constructed in accordance with the principles of - ;
: the present inven.tion; ::.
~15 Figure 60 s a graph illustrating voltage waveforms ~.
: developed in the arrangement shown in Figure 59; ~ :~
Figure 61 is a graphical representation of a Laue ; plot;
Figurc 62 is a circuit diap,ram similar to Figure 59 but illustrating a modiflcation of the arrangement shown : in Figure 59;
Figure 63 is a graph similar to Figure 60 but .~ illustra~ing.the arrangement shown in Figure 62;
Figure 64 is a sectional view of an embodiment :~
according to the three-phase glow discharge heating .
apparatus of the present invention and a diagram of a : :
control circuit therefor;
Figure 65 is a graph illustrating various waveforms :~

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I , developed in the arrangement shown in Figure 64;
Figure 66 is a diagram of the detailes of the control circuit shown in Figure 64;
Figure 67 is a wiring diagram of a modification of 1 the arrangement shown in Figure 66;
Figure 68 is a graph similar to Figure 65 but illustrating the arrangement shown in Figure 67;
¦ Figure 69 is a longitudinal sectional view of a I modification of the arrangement shown in Figure 44 and a ¦ diagram of a control circ~it therefor;
Figure 70 is a longitudinal sectional view of a modification of the arrangement shown in Figure 69;
¦ Figure 71 is a view similar to Figure 64 but illustrating a modification of the arrangement shown in ¦ Figure 64;
¦ Figure 72 is a view similar to Figure 70 but illustrating a modification of the arrangement shown in ¦ ~igure 69; and ¦ Figure 73 is a longitudinal sectional view of ~20 ¦ another ~odification of the arrangement shown in Figure 69.
Throughout the Figures like reference numerals designate the identical or corresponding components.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
I , _ ¦ Referring now to Figure l of the drawings, there is 1 illustrated a conventional glow discharge-heating apparatus.
The arrangement illustrated comprises a hollow cylindrical cathode electrode 1, z hollow cylindrical anode electrode 2 surrounding coaxially the cathode electrode 1 to form an ~ 3Z~5 I .
annular discharge gap 8 therebetween with the aid of two electrically insulating spacers 7 in the form of annuli fixedly disposed between both electrodes 1 and 2 adjacent to both end portions of the anode electrode 2, and a cylindrical enclosure 9 formed of any suitable electrically insulating material such as glass and coaxially housing the electrodes ,~
1 and 2 with the cathode electrode 1 hermetically extending through both ends thereof. A seal fitting 10 is sealed at the outer periphery to one end, in this case, the lefthand end as viewed in Figure 1 of the enclosure 9 and at the inner periphery to the adjacent portion of the cathode electrode 1 while a corrugated seal fitting 11 is sealed at the outer pheriphery to the other end of the envelope 9 and at the inner periphery to the adjacent portion of the cathode electrode 1. The corrugated seal .
fitting 11 is permitted to be axially contracted and expanded enough to prevent the cathode electrode 1 from damaging due to an axial thermal strains thereof. Thus the envelope 9 and ~he seal fittings 10 and 11 maintain the discharge gap 8 hermetic.
As shown in Figure 1, the anode electrode 2 includes flared end portions 2g in order to prevent an electric -~
discharge from concentrating on the end portions of the anode electrode 2.
A positive terminal 5 connected to the central portion of the~anode electrode 2 is extended and sealed -through the central portion of the cylindrical peripheral wall of the enclosure 9 until it is connected to a positive æZ~5 . side of a source o DS voltage 3 having a negative side connected through a stabilizing resistor 4 to a negative terminal 6 that is, in turn, connected to one end portion, in this case, the ri~hthand end portion as viewed in .
Figure 1 of the cathode electrode 1.
In the arrangement of Figure 1, a DC voltage is applied across the anode and cathode electrodes 1 and 2 respectively to establish a glow discharge across the discharge gap 8 thereby to heat the cathode electrode 1. `~:
Under these circumstances, a ].iquid to be heated such as water is caused to flow through the interior of the cathode electrode 1 to be directly heated by the heated cathode electrode 1.
: Conventional glow discharge heating apparatus such : 15 as sh~wn in Figure 1 have been enabled to instantaneously heat liquids to be heated, for example, water resulting in heating apparatus simple in construction and still high in :~
efficiency. ~owever~ since the apparatus have re~uired the hi.gh current, it has been extremely difficult to stably sustain the glow discharge across the anode and cathode .:
:~ electrodes. According to circumstances, there hac been a fear that the glow discharge transits to an arc discharge. -Further there has been a fear that, as a result of their ~:
heating, the electrodes are axially expanded leading to the destruction of the heating apparatus. In addition, conventional glow discharge heating apparatus have not ~ :.
. been provided with suitable control circuit means for : st~rti~gand ceasing the glow discharge with the result that a~r~ 2~

it has been difficult to reliably control the glow discharge.
The present invention contemplates to eliminate the disadvantages of and objections to the prior art practice as above described and characterized by unique means for imparting a positive resistance to the current-to-voltage : characteristic of the glow discharge. It has been found that such characteristic is effective for pre~Tenting the ¦ :
; transit of the glow discharge to an arc discharge.
For a better understanding of the principles of the .
present invention, the description will now be macle in conjunction with the glow clischarge, the principles that it heats an associated cathode electrode and the current-to- ¦
voltage characteristic thereof.
Figure 2A shows a pair of cathode and anode ¦
lS electrodes 1 and 2 respectively disposed in spaced opposite relationship and a source of DC voltage 3 including a ¦ :
negative side connected to the cathode electrode 1 and a I :
positive side connected to the anode electrode 2 through a ¦
stabilizing resistor 4 whereby a glow discharge occurs within a discharge space formed between both electrodes 1 and 2. It is well known that the discharge space having the glow discharge established therein is divided into a region of cathode fall a in which positive ions are enriched, a region of negative glow b forming a thin lumunescent layer, a Faraday dark space c in which no light is emitted, and a pnsitive column do consisting of a plasma inc].uding electrons and ions, starting with the side o:f the catllode electrode 1.

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l Pigure 2B shows a spatial voltage profile in the ¦ discharge space with the glow discharge established therein.
In Figure 2B, a voltage V is plotted in ordinate against a l distance d in abscissa measured from the cathode electrode 1. ~`
S ¦ From Figure 2B it is seen that the region of cathcde fall l a has a very large potential-gradient because the presence ¦ of a space charge until a cathode fall of potential Vc is l reached at the end of the region a spaced from the sur-face I I of the cathode electrode 1 by a distance of dc. The ¦ voltage reaches a glow voltage Vg on the surface of the anode electrode 2.
l By visually observing the g}ow discharge, it is seen ¦ that a boundary bctween the region of cathode fal:L a and ¦ the region of negative glow b are very distinct but a ¦ boundary bet~een the region of negative glow b and the ¦ Faraday dark space c or between the Faraday dark space c ¦ and the positive column d~ is not so distinct.
¦ AIso the Faraday dark space c and the positive ¦ column do are in the so-called plasma state and relatively 1 small in potential gradient. On the other hand, the region of cathode fall a includes positive ions in the form of a beam. As far as the discharge current is concerned, it consists essentially of an electron current in each of the ¦ Faraday dark space c and positive column do which are in Z5 ¦ the plasma state and of an ion current in the region of cathode fall a. The region of negative glow forms a region of the transition of one to the other of both currents.

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-~) Two phenomena developed in the region of cathode fall a, that is, ~1) the mechanism by which ~he glow discharge is sustained and (2) a phenomenon that the cathode electrode is heated with the glow discharge as well as (3) the current-to-voltage characteristic of the glow discharge are pertinent to the principles of the present invention and therefore will now be described.
(l) Mechanism of Sustaining Glow Discharge Positive ions present in the region of cathode fall a collide against the surface of the cathode electrode whereupon the cathode electrode 1 emits electrons by means of the action of emitting secondary electrons called the Yi action, The elcctrons emitted from the cathode electrode (1) collide against neutral atoms or molecules during their movement toward the anode electrode which is accompanied by an ionizing action called the ~ action with some probability. Electrons and positive ions caused by the ionization and collision are accelerated toward the anode and cathode electrodes respectively by means o~ the ; 20 action of an electric field involved. It is noted that the positive ions accelerated with the electric field contributes to the Yi action.
Here .the sustainment of the glow discharge will be somewhat quantitatively described. For example, it is said that, with the cathode electrode l formed of nickel~ the Yi is appro~imately equal to 0.01 for slow helium ions having 1 Kev or less. That is) about 100 ions collide against the cathode electrode 1 ~o emit a single electron r~2 therefrom. .
Also a degree of ionization ~ is a function of the type and pressure of a gas confined in the discharge space l and a potential gradient developed therein. Electron-ion ¦ pairs formed at a distance x from the cathode electrode 1 are proportional to eaX where e designates the base of Napierian logarithms and therefore increa.se exponentially ¦ with the distance x. Accordingly? the glow di.scharge is I I sustained with a distance and a voltage required for about `:
¦ 100 electron-ion pairs to be formed in the course of :::
movement of a single electrode toward the anode electrode 2. This dlstance is designated by the distance dc shown l in Figure 2B and this voltage substantially corresponds to 1 the voltage Vc. In other words, the glo-~ discharge can be ¦ sustained even when the anode electrode 2 has been displaced to its position substantially shcwn by dc in Figure 2A.
This is substantially applicable to electrodes formed o~ nickel, copper, iron, stainless steel or the l like and operatively associated with a gas selected from ¦ the group consisting of helium, neon, argon, hydrogen, nitrogen etc.
A more detail~analysis of the phenomena developed in the vicinity of the cathode electrode 1 teaches that a I current density J on the surface of the cathode electrode 1 ¦ is expressed by , j+ ~ j = j+ = KlP2 ~1) .,ll 1:
where i+ and i designate densities o-f positive ions and electrons respectively, P a pressure of a discharge gas, and Kl designates a constant determined by both the type of a cathode material and that o-f the discharge gas.
Also the region of cathode fall a has a thickness dc as defined by . . ~;
dCP = K2 (2) where K2 designates a constant dependent upon both the type of the cathode material and that of the discharge gas.
Within the region of normal glow, the cathode fall of potential Vc is determilled by both the type of the cathode material and that of the discharge gas but scarcely depends upon both a discharge current and the pressure of the discharge gas.
The following Table I lists values of the constants Kl and K2 and the cathode fall of potential Vc measured within the region of normal glow with different combina-tions of cathode materials and discharge gases with a glow current not higher than 1 amperes and with the discharge gases maintained under the pressure of 50 Torrs or more.
The measured Kl and K2 values are expressed in 10 6 ampere per cm2Torr2 and in cm Torr and the voltage Vc is expressed in volts. Also the current density on the surface of the cathode electrode has beer. determined by measuring an area of a negative glow b. Probably~ the negative glow is very thin so that it is observed like a ~ 5 luminescent film attached to the cathode electrode.
Table I
~IEASURED VALUES QF Kl, K2 and Vc ~ Gas He Ne Ar H
Cathode ~~~- _ 2 _ -~ _ . , - . _ . - -Kl 8 3 27 .
Cu 2 3 0 3 0 0 8 2 0 ~!:
c _ 150 150 1~0 290 Kl 8.0 20 32 3 Ni 2 1 5 3 0 Vc 101 140 185 254 _ .~ __ Kl 30 Mo 2 3 0 0.8 3 0 ~ --~-- vc - 180 175 190 290 ¦ SUS 2 5~7 0 8 1 5 _ Vc 119 150 180 232 ~`
_ ._ (2) Heating of Cathode ~lectrode As above descrlbed, positive ions present in the region of cathode fall a collide against the cathode electrode to cause the Yi action. At that time, the posi-tive ions have surplus kinetic energy that is, in turn spent to heat the cathode electrode 1~ Regarding auantities of input and output heat of the cathode electrodes, there are, in addition to collision with the positive ions, heat I .
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conduction from the plasma portions, exothermic and endothermic effects caused from chemical reactions effected on the surface of the cathode electrode 1 due to the glow discharge, cooling effects caused from the sputtering on the cathode electrode and the evaporation of the cathode ma~erial etc. However, an extent to which a quantity of heat enters the cathode electrode has not been elucidated until the present.
In order to determine a quantity of input heat to the cathode electrode due to the glow discharge formed between that electrode and an anode electrode, experiments were conducted with a test device schematically shown in ~igure 3. As shown in Figure 3, a cathode electrode 1 in the -form of a very long circular rod having a radius r of lS 1.8 mm was disposed to be thermally isolated from the surrounding and opposite to a similar anode electrode 2 to form therebetween a gap having a length d of 4 mm.
Both electrodes were formed o copper and connected across a DC source 3 through a stabilizing resistor 4. Thus a glow discharge g is established across both electrodes 1 and 2 in the atmosphere. Under these circumstance, a radiation thern.ometer M was used to continuously measure a temperature at a point on the outer surface of the cathode electrode 1 spaced way from the discharge surace thereof by a distance ZO of 3 mm.
The results of the experiments are shown in Figure 4 wherein the temperature in Centigrade is in ordinate against time in seconds in abscissa with a glow current taken as ,.1~

3~ 245 the parameter. In Figure 4, each vertical seginent designates a range in which measured values of the temperature are dispersed and solid curve describes calculated values of the temperature as will be described hereinafter. The reference numerals 111, 112, 113 and 114 mean the tempera-tures measured and calcu]ated with glow currents of 4~0, 250 200 and 150 milliamperes respectively.
; From Figure 4 it has been confirmed that the glow discharge ~ransits to an arc discharge upon the measured temperature approaching 1000C. This will be because an oxide film is formed on the surface of the ca~hode electrode at such a temperature.
It is now assumed that in Figure 3, the cathode electrode l l~ith a radius r has the longitudinal axis lying on a z axis and the discharge surface passing through the origin for the z axis and that a quantity of input heat to the cathode electrode 1 is constant per unit area and per unit time. Under the assumed condition, by solving a partial differential equation for conduction of heat re-ferred to the z axis alone and taking account of a ~;
radiat;on loss may be expressed by ~T = ~2 a _~_ a (T - To) where K designates a thermal diffusibility defined by the square root of the quotient of a ~hermal conductivity k of the cathode electrode divided by the product of a density p and a heat capacity thereof ~nd a is a cons~ant on the ,~ ;

~ 2~ 5 assumption that the radiation loss is a linear function of a temperature T. By solving the partial equation under the boundary conditions s aT I lvc ~Vc and az ¦ =

I ~
: where ~ designates a coefficient of heat input and the initial condition 1~ ~(z,~) = To where To designates room temperature, a solution results in ~ T(z,t) lo ~r2k ~ ~ e (I -~ F(yl)) - e F(y~) J (3) 2S .
where I: glow current.
In the expression (3) F(yl) and F(y2) are error functions expressed by .ll .

~ 2~i ; 2 ! , F(y~ ; e 2 dy and F~y2~e Z dy respectively where Yl and Y2 are expressed by ~
2K~t-z 2 K ~t + Z

respectively. Also ~ is defined by 2Ea ~Ta3+ToTa2+To2Ta+To3) oL = ~ .,:-pca .

where E designates an emissivity, a a Stefan-Boltzmann constant and Ta designates the mean value of room tempera-ture and a temperature of the cathode electrode.
~he expression (3) was used to calculate the time dependency of ~he temperature rise on the measured point as shown in Figure 3. The results of the calculations are indicated by the solid curves shown in Figure 4.
From Figure 4 it is seen that the measured values of the temperature fairly well coincide with the calculated values thereof.

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¦ Figure 5 illustrates a glow discharge voltage V in ¦ volts plotted in ordinate against a length of a discharge gap in millimeters in abscissa. The vo:Ltage V was measured ¦ with the electrodes formed of copper and disposed in the ; 5 ¦ atmosphere. Curves labelled llS, 116, ll7 and 118 depict ¦ glow currents of 10, 50, 100 and 400 milliamperes respec-~; ¦ tively.
In Figure 4 it is to be noted tnat the curves have l been drawn by equalling the cathode drop of potential Vc ¦ in the atmosphere to a voltage of 285 volts estimated with ¦ a null gap length from curves shown in Figure 5.
Also the coefficient of heat input ~ has been determined to cause the calculated valves of the temperature l to coincide with the measured values thereof shown in lS ¦ Figure 4. The coefficient ~ has been of 1.4.
Further it is considered that a quantity of heat corresponding to 0.4 jVc per unit area per unit time will result from one portion of heat generated in that portion of the glow discharge formed of both the Faraday dark space c and the positive column d except for the region of cathode fall a having a thickness dc approximately equal to 2 x 10 3 centimeter.
Figure 6 illustrates a glow voltage Vg in volts plotted in ordinate against a glow current I in milliamperes in abscissa. Curve labelled ll9 describes the glow current-to-voltage characteristic exhibited by the arrange-ment o Figure 2. Dotted curve 120 shows the total power consumed by the glow discharge and expressed by IVg while ,-. ~ !

3L~ 45 broken curve 121 illustrates an electric power entering the cathode electrode and calculated as 1.4 IVg. Both the glow voltage and powers in watts are plotted in I ordinate against the same glow current in absussa.
S ¦ From Figure 6 it is seen that at least 80 % of the total consumed power enters the cathode electrode and that the higher the glow current I the greater the proportion of the power entering the cathode electrode to the total ¦ consumed power will be.
1 Also it is seen that a quantity of input heat q to the cathode electrode 1 per unit area per unit time is give by I q= iVC = jVg ~ ~
provided that the spacing d between the cathode and anode electrodes 1 and 2 respectively substantially approximates I the thickness of the region of cathode fall a ~see Figure ¦ 2~, that is to say, the glow discharge includes no plasma ¦ portion. From this it is seen that the smaller the spacing d between the cathode and anode electrodes the larger the proportion of the power entering the cathode electrode to the total consumed power will be.
Figure 7 shows a model for a positive ion flux striking against the unît area of the surface of the cathode electrode per unit time. In Figure 7, a square prism has a square bottom including each side of 1 centi-meter and contacting the surface of the cathode electrode ,,Ll ~ 2 ;, I
¦ 1 and a height corresponding to the velocity Vi cm/sec of ~ ions multiplied by one second. Within the prism, positive ¦ ions designated by the symbol "cross in circle" are moved as shown at the arrow to strike collide with the cathode ¦ electrode 1. Thus the square prism designates a positive ion flux colliding against the cathode electrode per unit ¦ area per unit time and electrical energy o-f the ion flux ¦ results in the quantity of input heat q to the cathode ¦ electrode. Since the number of the positive ions is ¦ expressed by j/e where a designates the elementary electric ¦ charge and since each ion has electrical energy of eVc, the ¦ quantity of input heat ~ is expressed by l q = eVc ~ = jVc in watts/cm2.
¦ Thus the model for the positive ion flux also ¦ explains that the quantity of input heat to the cathode I electrode is expressed by jVc per unit area per unit time.
¦ From the foregoing it will be understood that the ¦ glow discharge established across the cathode and anode ¦ electrodes causes the quantity of heat expressed by ~JVc ¦ to enter the cathode electrode per unit area per unit time.
Also by decreasing the spacing between both electrodes to ¦ increase the glow current through the spacing, the ¦ quantity of inpu~ heat to the cathode electrode per unit area per uni~ time can approximates the product of the ¦ current density on the surface of the cathode electrode ¦ multiplied by the glow voltage or J-Vg.

l~ 2~5 There~ore the glow discharge without the positive column can be utilized as a heat source having a high efficiency because almost all heat due to the glow discharge enters the cathode electrode and also as a heat source having a power density variable at will by changing a gas pressure within the spacing between both electrodes because the current density on the surface of the cathode electrode is proportional to the square of the gas pressure.
(3) Current-to-Voltage Characteristic of Glow Discharge The current-to-voltage characteristic of the glow discharge will now be described and then the principles of the present invention will be described in detail.
Figure 8 shows the relationship between a current and a voltage for the glow discharge. In Figure 8 the axis of abscissas represents a current and the axis of ordinates represents a voltage.
A DC voltage is applied across a cathode and an anode electrode 1 and 2 respectively (see Figure 9A) to render the anode electrode 2 positive with respect to the cathode electrode 1 thereby to cause to a glow discharge thereacross. I~'hen a current flowing through both electrodes is increased, a negative glow region b included in the glow discharge spreads in area on the surface of the cathode electrode 1 (see Figures ~A and 9~). This results in a change in current-to-voltage characteristic as shown at solid line N in Figure 8.
However~ when the current is quite low, the z~

¦ current-to-voltage characteristic droops as shown by a characteristic portion Nl in Figure 8. A region in which the drooping characteristic Nl appears is called a region ¦ of subnormal glow e.
¦ In a region following the region of subnormal glow ¦ e an increase in current causes the voltage to be Xept ¦ substantially constant as shown by a characteristic portion ¦ N2 in Figure 8 as long as that the surface of the cathode ¦ electrode 1 having the negative glow b caused thereon is ¦ smaller in area than the entire surfac0 thereof opposite ¦ to the anode electrode 2 as shown in Figure 9A. A region ¦ in which the characteristic portion N2 is developed is ¦ called a region of normal glow f.
¦ A further increase in current causes an increase in ;~
lS ¦ voltage because the nega~ive glow _ has covered the entire ¦ area of the surface of the cathode electrode 1 opposite to ¦ the anode electrode 2 as shown in Figure 9B whereby the ¦ negative glow increases in current density. The resulting ¦ I-V characteristic is upturned with an increase in current ¦ as shown by a characteristic portion N3 in Figure 8. The ¦ characteristic portion N3 is called a positive resistance ¦ characteristic and a region in which the positive resistance characteristlc N3 appers is called a region of abnormal I glow. In that region of abnormal glow q the entire area ~ of the surface of the cathode electrode 1 is covered with ¦ the negative glow _ ~see Figure 9B) with the result that ¦ the current is apt to concentrate at the edge portion or ¦ the li~e of the cathode electrode 1 and therefore the glow :
::

discharge is easily changed to an arc discharge. As a result, it is difficult to maintain the glow discharge in its stable state. The arc discharge appears in a region h as shown in Figure 8.
With no impedance connected between the cathode and anode electrodes 1 and 2 respectively and an electric source for supplying an electric power across both electrodes, the source side has the current-to-voltage characteristic of the constant current type such as shown at horizontal ~10 broken line P in Figure 8. This is because even an increase in current does not cause a voltage drop across an impedance.
Under these circumstallce, the glow discharge has its operating point coinciding with a point Pl where the characteristic P of the source side intersects the charac-teristic N of the glow discharge. Ilowever, this operating point Pl is located in the region of abnormal glow g, which is apt to transit to a region of arc discharge h, as above described. Accordingly it is difficult to maintain the -~
; 20 glow discharge stable in the region of abnormal glow g.
Further it is to be noted that the flat character-istic P of the source side can not stably cross the flat characteristic portion N2 of the glow discha-rge in the region of normal glow f.
On the other hand, Wit]l a resistance R as the impedance connected to the source, an increase in current I
causes an increa$e in voltage drop IR across the resistance.
, :, .
Thus the source side has the current-to-voltage characteristic t I ~3~3Z~5 such as shown at dotted straight line Q in Figure 8 and the glow discharge has its operating point designated by an intersection Ql of the characteristics Q and N. This I operating point is located in the region of normal glow f ¦ resulting in the stable glow discharge.
Where electrical energy participating in the glow discharge is converted to thermal energy with a very high efficiency, the connection of an impedance to the source as l above described forms one of factors of decreasing an ¦ efficiency utilization of electrical energy. For example, the use of a resistor causes a Joules loss and the use of reactor causes a Joules loss of a winding involved and an eddy current loss and a hystersis loss of an iron core l involved. Since such energy losses scatter as thermal 1 energy, it is possible to recover the thermal energy.
This, of course, deprives the resulting heating device of its convenience and compactness.
From the foregoing it is seen that whether or not an l impedance is connected to an electric source retains the 1 abovementioned disadvantages as long as the glow discharge has the current-to-voltage characteristic in the form of a curve such as shown at N in Figure 8.
In order that the glow discharge can be maintained l stable even with the flat current-to-voltage characteristic ¦ of an associated source side such as shown by straight line P in Figure 8 and without an impedance connected to the source, the present invention includes unique means for imparting a positive resistance to the current-to-voltage I

,_ll ~ 2~
l, characteristic of the glow discharge in a different manner as compared with conventional abnormal glows.
First it is seen in Figure lOA that surface of a cathode electrode 1 opposite to an anode electrode 2 S has an area made sufficiently larger than that of the anode electrode 2 so as not to impede the spread of a negative glow A b. In other words, the opposite surface area of the anode electrode 2 is limited to a small magnitude with respect to the cathode electrode. Thus a peripheral edge root bl of the negative glow _ lying on the opposite surface of the cathode electrode 1 has a distance to the anode electrode 2 is gradually increased as the negative glow _ spreads due to an increase in glow discharge current and therefore a voltage across both electrodes 1 and 2 is gradually raised. Under these circumstances, the glow discharge has the current-to-voltage characteristic such that the voltage increases with the current as shown at broken curve T in Figure 8.
That is, the characteristic is of the positive resistance type.
In this connection, it is to be noted that the positive resistance characteristic dev01Oped in the region of abnormal glow g in the prior art practice as shown at curve N3 in Figure 8 is cause from the fact that the negative glow b has spread o~er the surface of the cathode electrode and can not any more spread (see Figure 9B).
Accordingly, such positive resistance characteristic is ~uite different from that according to the principles of ,~"l ~

- 'L~ .'P'~ i the present invention. As above described, the negative glow of the present invention is permitted to sufficiently spread as an increase in current because the active surface area of the cathode electrode 1 opposite to the anode electrode 2 is suficiently larger than that of the anode electrode with the result that there is no problem that the glow discharge is transits to an arc discharge due to the impossibility of spreading the negative glow.
From the foregoing it is seen that the characteristic T of the present invention as shown in Figure 8 is developed in the region of normal glow but ]lOt in the region of abnormal glow although it has a positive resistance.
In the present invention, even with an associated electric source having no impedance connected thereto, lS therefore the characteristic T thereof intersects the characteristic of an associated source side at a point Tl (see Figure 8) where the glow discharge is stablized. It is to be noted that the point Tl lies in the region oE the normal glow unlike tlle characteristic N3 of the prior art practice so that the present invention does not encounter the problems that the glow discharge transits to an are discharge and so on.
In order to impart a positive resistance to the c~rrent-to-voltage characteristic of the glow discharge by further increasing the distance between the peripheral edge bl of the negative glow h on a cathode electrode 1 and an associated anode electrode 2, the cathode electrode 1 c~n be made cylindrical and opposite to the anode electrode ,11 -~ l ~ 5 I ..
` I
¦ 2 as shown in Figure lOB. In the arrangement of Figure lOB, l the peripheral glow edge bl is located on the peripheral ¦ wall surface of the cylindrical cathode electrode 1 at some --¦ distance from the end surface thereof. Thus, the glow edge ~¦~
I ~1 is far spaced away from the anode electrode 2 as compared ¦ ~ith the arrangement of Figure lOA, resulting in a satisfactory positive resistance characteristic.
When an AC voltage is applied across the cathode and ¦ anode electrodes, either of the electrode becomes alternately 11:
¦ a positive electrode so that a glow discharge is caused on ¦ the opposite surfaces of both electrodes. With an AC ~ .
¦ voltage used, it is desirable that the cathode and anode r electrodes are in the -form o~ identical cylinders and oppose I to each other as shown in Figure lOC. From Figure lOC it ¦ is seen that the peripheral edge bl of the negative glow b ¦ on either electrode 1 or 2 is far spaced away from the mating ¦ electrode 2 or 1 as in the arrangement of Figure lOB.
From the foregoing it is summarized -that the I principles of the present invention are to cause an area ¦ with which a pair of cathode and anode electrodes are ¦ opposite to each other to be smaller than that area of the electrode with which a negative glow is caused.
Referring now to Figure 11, there is illustrated I one embodiment according to the glow discharge heating ¦ apparatus embodying the principles of the present invent~on ¦ as above described. The arrangement illustrated comprises ¦ an electrically insulating enclosure 9 in the form of a ¦ hollow cylinder formed of glass, a cathode electrode 1 in the ~, S

¦ form of a hollow cylinder with both open ends coaxially ¦ extending through the enclosure 9 and an anode electrode 2 ¦ in the form of a hollow cylinder with both open flare ends ¦ disposed coaxially ~ith the cathode electrode 1 withing ¦ the enclosure 9 to form an annular glow discharge gap 8 therebetween. The cathode electrode l is extended and ¦ sealed through both ends of the enclosure 9 by means of ¦ seal fitting 10 and 11 respectively. Thus the enclosure 9 ¦ along with the cathode electrode 1 defines an annular space ¦ 81 which includes the glow discharge gap 8 and is filled with an electrically dischargeable gas selected from the group consisting of rare gases such as helium, mixtures thereof, for example, a mixture of neon and argon, a mixture of helium and hydrogen etc.
An annular anode terminal 5 is fixedly secured at the inner periphery to the central portion of the outer cylindrical surface of the anode electrode 2 and has a protrusion extended and sealed through the enclosure 9 by having the outer periphery fixed to a seal fitting sealed to adjacent ends of two similar enclosure portions forming the enclosure 9. The anode terminal 5 is connected to a positive side of a source of DC voltage 3 including a negative side connected by a stabilizer 4 to a cathode terminal 6 that is connected to that portion of the cathode electrode 1 dispose~ putside of the enclosure 9, in this case, adjacent to the seal fitting 11. The stabilizer 4 may be a small capacity reactor or a resistor. If desired, the stabili r may be omitted.

: :

.. . .. .. . . ... . . .

I ~

In order to -facilitate the description of the present invention, the symbol 'IS-'l designates the entire area of that portion of the cathode electrode 1 on which a glow discharge can be caused while the symbol IlS~ll designates an area of that portion of the anode electr~de 2 opposing to the cathode electrode 1 and actually used for the glow dis-; charge. Therefore an area labelled S+ l5 called an "anode area effective for discharge" or an "effective anode area".
According to the principles of the present invention as above described, the anode area S+ effective for discharge is made smaller than the cathode discharge area S-.
The operation of the arrangement as shown in Figure 11 will now be described. A DC voltage from the source 3 is applied across the anode and cathode electrodes 2 and 1 respectively through the stabilizer 4 to establish a stable glow discharge in the annular discharge gas 8 thereby to heat the cathode electrode 1 Under these circumstances, a fluid to be heated, for example, water flows into the interior of the cathode electrode 1 as shown at the arrow A
in Figure 11 to absorb heat from the cathode electrode 1 to be heated. Then the heated fluid flows out from the catnode ~lectrode 1 as shown at the arrow B in Figure 1.
During the glow discharge, a current and a voltage ~ ;
thereof is illustrated by a characteristic curve shown in Figure 12 wherein the glow discharge current Ig in amperes is plotted in abscissa against the glow discharge voltage Vg in volts in ordinate. The glow discharge voltage Vg may be approximately expressed by _,~,, ll .~ S
Vg = VO + IgR

where VO designates a glow discharge-hold minimum voltage as will be described hereinafter and R designates a slope S of the characteristic curve. The slope of the characteristic curve as shown in Figure 12 is called a l'positive resistance Referring back to Figure 11, L designates an axial ~ength of the anode electrode 12 and has been differently ;~
; changed to vary the effective anode area S+ thereby toobtain the relationship between a ratio of the effective anode area S~ to the cathode discharge area S- and the positive resistance R as shown in Figure 13.
In Pigure 13, the positive resistance R in ohms is plotted in ordinate against the ratio between both area S~/S- in abscissa. Curves labelled 122, 123 and 124 have been plotted with data measured by filling the interior of the enclosure 9 or the annular space 81 with a gaseous mixture including 70% by volume of helium and 30% by volume of hydrogen under pressures of 1~0, 150 and 200 Toors respectively. The gap between both electrodes 1 and 2 has heen maintained at a magnitude of 1 mm. Also the vertical segment has the same meaning as that shown in Figure 4.
From Figure 13 it is seen that the positive resist-ance R at the ratio of S+/S- of 0.2 increases to four or four times that at the ratio of 1.
The tendency of the positive resistance characteristic as shown in Figure 13 can be observed with the spacing of ,~:
~ :

, ,11, I ~

5 mm between the both electrodes 1 and 2 filled with the dischargeable gas including neon, helium, a mixture of neon and argon, or a mixture of helium and at most 30% by volume ¦ of hydrogen under a pressure of 200 Torrs or less.
¦ Also experiments have been conducted with the DC
source 3 having varied regulations of the source voltage.
The results of the experiments are shown in Figure 14 wherein the axis of ordinates represents a regulakion of source voltage in percent and the axis of abscissas represents a ratio of the actual discharge current I to a rated discharge current Io in percent. Straight lines labelled 125 and 126 describe the regulations of source voltage with the positive resistance R having value of 1 and 3 ohms respectively.
From Figure 14 it is seen that a variation of 15~
in source voltage gives a current regulation or a ratio of ; the actual current I to the rated discharge current Io multipled by one hundred in percent ~42% and ~14% with positive resistance R of 1 and 3 ohms respectively. Thus the positive resistance R of 3 ohms renders the glow discharge relatively stable.
Further by rendering the positive value R higher, it is possible to control a maximum current for supplying a predetermined.electric power to a small magnitude which is, in turn, advantageous in that the glow discharge apparatus is made compact.
The measure as above described is also applied to constructions in which the AC voltage is applied across the e~ectrodes 1 and 2 to cause the glow discharge thereacross 2~ 5 only when the electrode l acts as a cathode electrode.
From the -foregoing it is seen that the arrangement of Figure 11 eliminates the disadvantages of conventional glow discharge heating apparatus that the positive resistance S for the glow discharge is low, the glow discharge is moved about on the electrode, the discharge current much changes with a variation in source voltage resulting in the necessity of connecting a stabilizer or like to the source and so on. Tl-ose disadvantages have been caused from the cathode area substantially equalling the anode area.
~igure 15 shows a modification of the present invention operatively associated with an AC source. The arrangement illustrated comprises an inner electrode 1 in the form of a hollow cylinder having one end closed, an cuter electrode 2 in the form of a hollow cylinder having cne end open and disposed coaxially with the inner electrode l so that the closed end portion of the inner electrode l is inserted into the opened end portion of the outer electrode 2 to form an annular discharge gap 8 therebetween.
The inner electrode 1 is coaxially disposed within a tubular glass enclosure 9 to extend beyond both open ends thereof and the open end portion o the electrode l is rigidly fitted into an annular supporting disc l3 of any suitable metallic material including an outer periphery connected to the adjacent end of the enclosure 9 through the seal ~itting 11. The outer electrode ~ has the open end portion extending into the enclosure 9 and supported to another annular supporting disc 14 of the same material as ,,11 ' ` ~

82~5 l the disc 13 similarly connected to the other end of the ¦ enclosure 9 through another sela fitting 10. In ~his way the enclosure 9 defines a her~etic space 81 with the I supporting discs 13 and 14~ the seal fittings 10 and 11 9 ¦ the inner electrode 1 and the outer electrode 2 having ¦ the other end closed.
I Then a pair of terminals 5 and 6 is attached to the ¦ supporting discs 13 and 14 to connect both electrodes 1 I and 2 to an AC source 3] therethrough.
¦ An inflow tube 15 is coaxially disposed within the ¦ inner hollow electrode 1 to form an annular passageway ¦ therebetween. The tube 15 is maintained in place through a I closing member 16 rigidly fitted into the open end of the ¦ inner electrode 1 and having the tube 15 extending there-¦ through. The inner electrode 1 is provided on the open ¦ end portion with an outlet duct 17.
¦ ~n the other hand, the outer electrode 2 is double I ~alled and provided on the closed end portion of the outer ¦ wall with an inlet duct 18 and that portion thereof adjacent ¦ to the supporting disc 14 with an outl~t duct 19 communicat-¦ ing with the inlet duct 18 through an annular space defined ¦ by the inner and outer walls of the electrode 2. A liquid ¦ to be heated, for example, water enters the inlet duct 18 ¦ as shown at the arrow A in Figure lS and thence to the ¦ annular space due to the double-walled structure of the outer electrode 2 after which it leave the outlet duct 19. Also water enters the inflow tube 15 as shown at the arrow C in Figure 15 and thence the annular space between the inflow tube 15 and the inner electrode 1. Then the water flows out from the outlet duct 17 as shown a~ the arrow D in Figure 15.
It will readily be understood that the space 81 is filled wi~h an easy dischargeable gas as above descr;bed in ~' conjunction with Figure 11.
In operation an AC voltage across the source 31 is zpplied across both electrodes 1 and 2 to cause a glow cischarge mainly in the annular discharge gap 8.
As above described, the inner electrode 1 is inserted into the outer electrode ~ to overlap ~he latter.
This ensures that an area of that portion o-E one of the electrodes opposite to the other electrode is smaller than an electrode area with which a glow discharge can occur between the electrodes 1 and 2. This means that an anode -area on the side of that electrode acting as an anode for the glow discharge is always limited. For example, with the dischargeable gas maintained under a pressure of about 200 Torrs and with the gap between both electrodes having a length not exceeding 5 millimeters~ the limitation of the anode area resul~s in an indirect limitation of an associated negative glow reglon and therefore an increase in positive resistance fo,r the glow discharge. That is, the current-to-voltage characteristic of the glow discharge such as shown at curve Tl in Figure 8 has a larger slope whereby the AC
glow discharge can be maintained stable. Accordingly, a stable glow discharge can be sustained even with a high current under a high pressure without the glow discharge ,ll ~ 5 changed to an arc discharge.
Under these circumstances, a either of the inner and outer electrodes 1 and 2 respectively is heated when it acts as the cathode electrode resulting in heating of both electrodes. Thus the fluid such as water flowing in contact with the electrodes is instantaneously heated and the heated fluid leaves the outlet ducts 17 and 19.
Figure 16 is a characteristic curve illustrating the relationship between the area of one of the electrodes overlapping the other electrode and the positive resistance exhibited by the glow discharge. In Figure 16 the positive resistance R in ohms is plotted in ordinate against a ratio of the overlapping area to the entire area of ~he electrode acting as the cathode in abscessa. Curves labelled 127, 128 and 129 have been plotted with the discharge gap 8 having a length not exceeding 5 mm and filled Wit]l a mixture of helium and hydrogen under pressures of 100, 150 and 200 Torrs respectively. The vertical segment has the same meaning as that shown in Figure 4.
From Figure 16 it is seen that the smaller the overlapping area for both electrodes 1 and 2 the higher the positive resistance will be.
In the arrangement of Figure 159 the inner and outer electrodes 1 and 2 respectively are disposed in coaxial relationship but different in shape from each other. There-fore the curr~n~-to-voltage characteristic of the glow discharge is different between the half-cycle of the source 31 having the inner electrode 1 acting as a catl~ode and that ~0~ 15 having the outer electrode acting as an anode as shown in Figure 17. In Figure 17, the axis of ord;nates represents a discharge voltage V and the axis of abscissas represents ~;
a discharge current I. When the inner electrode 1 acts as the cathode, the discharge current I is forwardly and rearwardly changed along a straight line 130 shown in Figure 17 and has a maximum value of Il. In the next succeeding half-cycle the outer electrode 2 takes over the cathode and the current is forw~rdly and rearwardly changed along a straight line 131 shown in Figure 17. In the latter case, the current has the absolute maximum value I2 different from that of the current I2 flowing in the just preceding ;~
half cycle of the source 31. Both straight lines have the same absolute values of a voltage V0 at a null current.
Thus the resulting characteristic become unsymmetric to permit a zero-phase sequence component of a current to flow through the AC source 31. This is objectionable to the source 31. Further the inner electrode 1 is free at one end but the outer electrode 2 includes no free end. This results in the occurrence of thermal strains in the outer electrode 2 during the glow discharge.
These objections can be eleminated by still another modification of the present invention shown in Figure 18.
In the arrangement illustrated, a first electrode 1 in the form of a hollow cylinder having one end closed with a flat disc opposes to a second electrode 2 identical to the first ~;
electrode to form a discharge gap 8 having a predetermined spacing or gap length of _ between the opposite closed end ~ ~ ~ ` ::
I ~
l -~ I
¦ surfaces.
A flow confining tube 20 or 21 of the double wall type inserted into the second or first electrode 2 or l ¦ respectively includes a central tubular portion extending on ¦ the longitudinal axis of the mating electrode, a radially ~; I extended end wall to form :a predetermined gap between the same and the internal closed end surface of the electrode and a peripheral wall extending in parallel to the internal I peripheral surface of the latter to form also a predetermined ¦ annular gap therebetween. Each electrode 1 or 2 is provided on the open end portion with an outlet duct 18 or 17 communicating with the Elow path ormed therein while al~nular ¦ ~lind cover disc 23 or 22 is rigidly inserted into the annular I gap be~ween the peripheral surface of the electrode 1 or 2 ¦ and the outerwall of the tube 21 or 20 at the open end. The ¦ ~urpose of the flow confining tubes 20 or 21 is to cause a ¦ fluïd to be heated to enter first the central tubular portion ¦ as shown at the arrow A or C in Figure 18 and flow along ¦ the internal surface of the mating electrodes at an increased ¦ speed to enhance the heat transfer between the -fluid and the I electrode and also to enable the fluid to be instantaneously ¦ heated. The heated fluid then flows out from the outlet ¦ duct 18 or 17 as shown at the arrow B or D in Figure 18.
¦ Then the first and second electrodes 1 and 2 ¦ respectively are sungly fitted into individual supporting ¦ rings 14 and 13 which are hermetiGally connected to both ends ¦ of circular enclosure 9 through annular seal fittings 10 ¦ and ll In this way both electrodes 1 and 2 are supported llQ~2'a5 in cantilever manner to the supporting members 14 and 13 and the substantiall portions thereof are coaxially disposed within the enclosure 9 to form the space 81 that is then filled with a dischargeable gas such as previously described.
As in the arrangement shown in Figure 11 or 15, the AC source 31 is connected across the electrodes 1 and 2 through the terminals 6 and S connected thereto respectively.
In the arrangement o~ Figure 18 it is noted that those portions of both electrodes 1 and 2 superposing each cther as designated by the reference character 1 is made smaller in area than that portion of each electrode on ~hich the glow discharge occurs. In the example illustrated the glow discharge occurs on each of the electrodes 1 and 2 throughout the surface.
The arrangement of Figure 18 is characteri~ed in that the electrodes 1 and 2 formed to be symmetric abut against each other with the predetermined gap 8 formed therebetween. This results in the symmetric glow discharge characteristic as sho~ in Figure lg. In Figure 19 similar to Figure 17, the characteristics 132 and 133 are substantially symmetric and have respective discharge currents Il and I2 ;~
equal in the absolute value to each other. f;
Also,-as the electrodes 1 and 2 supported in contilever manner to the annular supporting discs 14 and 13 respectively, the electrodes are prevented from breaking due to thermal stains.
It will readily be understood that the gap 8 between both electrodes 1 and 2 should be dimensioned so that the ~ 2~i ¦ electrodes are prevented from contacting each other due to ¦ termal expansions thereof in operation.
As in the arrangement of Figure 15, an AC voltage l across the source 3~ is applied across the electrodes 1 and ¦ 2 to cause a glow discharge between the opposite surfaces lI
thereo~ while a fluid to ~e heated enters the interiors of the electrodes 1 and 2 as shown at the arrows A and C
in Figure 18. Then the fluid flows through spacing formed I between each electrode and the ~low confining tube 21 or ¦ 22 to be heated wi~h heat generated on the electrode l or 2 ¦ due to the glow discharge. ThereaEter the heated fluid ¦ 10ws out from each outlet duct 19 or 17.
¦ Pigure 20 illustrates a modification of the arrange-I ment shown in Figure 18. As shown in vertical section in ¦ Figure 20A, the electrodes l and 2 of the identical ¦ structure opposes to and somewhat offset each other to form ¦ a predetermined discharge gap 8 therebetween. As seen in I side elevational views of Figures 20B and 2Q~, the electrodes ¦ l and 2 are in the form of rectangular boxes and therefore I discharge surfaces thereof are rectangular and Elat. Then each electrode is provided on the rear surface with a pair of inlet and outlet tubes.
In other respects, the arrangement is substantially --l identical to that shown in Figure 18. The electrodes l and ¦ 2 include the discharge surfaces identical in shape to each other and are of the cantilever type so that the arrangement exhibits the same results as that shown in Figure 18.
In the arrangements of the present invention shown . ~ $~ : -~:
in Figures 15, 18 and 20 the electrode material and impurities such as metallic oxides included in the electrodes might be scattered in the discharge gap during the glow discharge and sticked to that surface portions of the enclosure g facing the electrodes 1 and 2. This sticking of such metallic materials to the enclosure might lead to not only a danger that the seal fitting 10 and 11 are short-circuit with earh other through the sticked materials but 21so to a fear that~ if the scattered impurities again adhere to the electrodes that the glow discharge will have transited to an arc discharge. ~, The present invention also contemplates to eliminate ' ~,he danger and fear as above described, by the provision c~f the arrangement shown in Figure 21. The arrangement ~' illustrated is different from that shown in Figure 18 only in that in Figure 21 a pair of annular shields 24 and 25 one for each electrode are disposed to surround the mating electrodes and face at least the internal surface portions of the enclosure 9 by having flare ends thereof fixedly secured to the internal surface portions of the enclosure 9 ~, , respectively. Each shield 24 or 25 includes the substan- ~ ' tial poltion parallel to the associated electrode and ending short of the adjacent annular supporting disc 13 or 14. The shields 24 and 25 may be of an electrically insulating or conductive material.
In operation when the electrode material and the impurities are emitted from the electrode 1 or 2 and scattered in the discharge gap, they are sticked ~o that surface of Il ' ~ z~s each shield 24 or 25 facing the associated electrode and prevented from adhering to that inner surface portion of the enclosure 9 covered with the shield 24 or 25. Also the shield is effective for preventing the scattered electrode material and impurities from again adhering to the associated electrode. ;~
The arrangement illustrated in Figure 22 is ;~
different from that shown in Figure 21 only in that in Figure 22 a pair of annular electrodes 26 and 27 are buried in thc annular shields 24 and 25 formed of an electrically insulating material respectively. Then a suitable voltage is applied to the annular electrode 26 and 27 w]lereby the scattered metallic materials are apt to adhere to the shields 24 and 25.
Figure 23 shows another modification of the arrange-ment illustrated in Figure 21. In Figure 23 the electrodes ~ -1 and 2 are in the form of hollow flat discs and disposed in opposite relationship to form the discharge gap 8 having a predetermined gap length of d therebetween.
The seal fitting 10 in the form of a short hollow cylinder has one end fixedly secured to the peripheral portion of that surface of the electrode 1 remote from ~l~e electrode 2 and the other end in the form of a ~lange to an enclosure portion 91 in the form of an annulus. Then an annular shield disc 28 of electrically insulating material is located between the annular enclosure portion 91 and the peripheral portion of ~he electrode 1 by having a fitting perpendicular to ~he same and connected to the outer 1 lOB.~

peripheral surface of the seal fitting 10. The sealing fitting 11, an enclosure portion 92 and a shield 29 identical to the components 10, 91 and 28 respectively are l operatively coupled in the same manner to the electrode 2.
¦ A toroidal metallic enclosure portion 93 of double L-shaped cross section is hermetically connected t^ the annular enclosure portions 9. and 92 to form a hermetically closed space 81 in the form of a toroid.
l As shown in Figure 23, a feed water tube 18 and a ¦ drain tube 19 project in spaced relationship from that surface of the elec-trode 1 remote from the electrode 2 and a pair of deflector or baffle plates 30 and 32 are disposed ¦ in the interior of the electrode 1 so as to direct a liquid ¦ to be heated toward the peripheral portion thereof and enter lS ¦ the fluid into the drain tube 19 after it has flowed along `~
l the heated surface of the electrode 1 to be heated. Also ¦ a feed water tube 18' and a drain tube 17 similarly project from the electrode 2 and a pair of baffle plates 33 and 34 l are similarly disposed within the hollow electrode 2.
¦ If desired, the shield 28 and 29 may be formed of any suitable metallic material. In the latter case, the shields 28 and 29 are suitably insulated from -tlle associated e]ectrodes 1 and 2 respectively.
l Further the present invention contemplates to prevent ¦ the occurrence of electric shock-accidents throu~h the heated liquid such as water.
The arrangement illustrated in Figure 24 is substan-tially similar to that shown in Figure 22 except for tlle ~ 2~S ::

provision of means for preventing the user from receiving electric shocks. As shown in Figure 24, the control tubular portion of the flow confining tube 20 or 21 is connected :::
to an electrically insulatin~ tube 37 or 38 that is, in~::
turn, connected to metallic inflow tube 41 or 42.
The outlet of the flow confining tube 20 or 21 is connected to connecting tube 35 or 36 subsequently connected to an electrically insulating tube 39 or 40 that is, in turn, connected to a metallic outflo~ tube 43 or 44.
The metallic tubes 41 and 43 are electrically connected together to ground as do the metallic tube 42 and ~4.
It has been found that an end-to-end distance lp between the central tubular portion of the flow con~ining ~ :~
tube and the inflow tube or between the connecting tube and the outflow tube, that is to say, a length of the insulating portion should be equal to or less than a predetermined magnitude dependent upon a voltage applied across the electrodes, a resistivity of the particular liquid to be heated, a cross sectional area of the tube etc;
The arrangement o~ Figure 2~ is operated as follows:
A switcll 45 is closed to apply an AC voltage from the source 31 across the electrodes 1 and 2. This causes the flow confining tubes 20 and 21, and the connecting tubes 35 and 3~ to be put at a certain potential relative to the ground ;~
potential. For example, in glow discharge heating apparatus having a discharge input o~ about 8 kilowatts, the AC source 31 is required to supply to the heating apparatus an AC

`I ~L31~;~iZ.5 voltage having the effective value of 200 volts so that the tubes 20 21 35 lalld 36 are ~ut at a ~oltagc llaving the effective value of 200 volts.
On the other halld the meta]lic inflow tubes 41 and 42 and the metallic outflow tubes 43 and 44 are connected to ground so that the particular liquid -flowing into or out from the extremities thereof is put at a null potential. This elsures that electric shock acciclents are prevented from occurring through the liquid.
More specifically the source voltage is applied across the el~ctrodes l and 2 to cause a glow discharge therebetween. Ileat generated during the glow discharge heats thc liq~lid. When the heated liquid ~low withill -the apparatus the same reaches any of the tubes 41 42 43 and 44 where it is put at the ground potential. This ensures that the user is maintained safe.
Under these circumstances tle electrodes 1 and 2 rapidly transfers heat to the liquid flowing within the interiors thereof to prevent the electrodes 1 and 2 from effecting an abnolmal temperature rise whereby the stable glow discharge is sustained.
llowever as a potential difference having the effective value of 200 volts occurs between the inflow and outflow tubes 419 42 and 43 44 and the confining and connecting tubes 20 21 an~ 35 36 the insulating tubes 37 38 39 and 40 must l.ave a dielectric strength withstanding a voltage having the effective evalue of 200 volts. In this connection it is required to consider a leaklge 82q5 I ..
~:
currcnt flowing to ground through tlle liquid, in addition ¦ to the surface status of thc insulating tubes.
I In the arrangement of Figure 24 applied to a water ¦l~armer operated with the source voltagc of 200 volts, the ¦ same is obligated to be provided with a ]eakage breaker.
¦ Leakage breakers are responsive to the leakage current in ¦ excess of the predetermined magnitude flowing through the ¦ inflow and outflow tubes 41, 42 and 43, 44 to ground to be l continuously operated to prevent the source voltage from ¦ being applied across the electrodes 1 and 2. Accordingly, ¦ it is required to impart to the length Qp of the insulating I portion a value sufficient to limit the leakage current to ¦ a certain value or less.
¦ Assuming that each of the insulating tubes 37, 38, ¦ 39 and 40 has a cross sectional area of flow path designated ¦ by S and a ligned to be heated such as water has a resistivity ¦ designated by p, the insulating portion presents a resistance I Q before the l;~uid exlressed by I RQ P S (4) I ', ~:' I Also assuming that each of the insulating tubes 37, 38, 39 ¦ and 40 has a surface resistance sufficiently large as l compared Wit]l the resistance of ~he li~uid, the leakage ~ current IQ may be expressed by ~ IQ = RQ = V~ pS = _Q _ Ql ~5) Z~

where V~ designatcs a voltage across the li(luid located in the insulating portion having the length of Q~. Accordingly, the leakag~ current IQ is inversely proportional to the length Qp with the voltage V~, the cross sectional area ~S and the resistivity p remaining unchanged. ~ :
Figure 25 a graph i~llustrating the relationship between the length Qp of the insulating portion and the leakage current IQ on the basis of the above two expressions

(4) and (5~ and with VQ = 200 volts, S = 0.636 square : ~.
centi.meters (which results from the insulating tubes 37, 38, 39 and ~0 havi]l~, the inside dianleter of 9 mill;mcters) and p = 1300 ollms-centimeter. The resistivity of 130() ohms-. centimeter is a m;nimum value of a resistivity of usable water as determined by the IEC standards. In Figure 25 the leakage current IQ in milliamperes is plot~ed in ordinate ~ :
against the length Qp of the insulating portion in cent:l- ~;
meters in abscissa. ` ::`~
; Assuming that the particular water warmer is : .
provided with a highly sensitive l.eakage breaker having a rated sensible current of 15 millialrperes, the breaker has a rated inoperative current of 7.5 milliamperes. In order :
to prevent this leakage breaker from being continuously operated due to a leakage current flowing through the .3 `; ~ ' : insulating portion, the length _p of the la~ter is necessarily :~:
: ~5 of at least 13 centimeters with used water having a resist- .:
ivity of 1,300 ohms centimeter as will be seen from the curve of ~igure 25. The expression (5) indicates that the : -.
length ~ langes with the leakage current, voltage, the - 52 - : ~`
:
~' I cross sectional area of the flow path and resistivity of ¦ the liquid. Ilowever, a length of the particular insulating I portion can be estimated as above described and in accordance ¦ with the rating of a given lea~age breaker, the source ¦ voltage, a resistivity of the particu]ar liquid and ~he ¦ cross sectional area of the flow path.
In the arrangement of Figure 24, the flow path of ¦ the heated liquid has been provided with the insulating ¦ tubes having thc required length while each of the insulating ¦ tubes has been connected at the extremity to the meta11ic ¦ inflow or out-flow tube that is connected to ground. Accord-ingly, it is ensured that the any electric shock accident can be prevented from occurring tllrougll a liquid involved and still one can eliminate the insulating treatment that electrode components are coated with an electrically insulating material. lhis results in simplified inexpen-sive apparatus and also the heat transfer from the electrode components to the liquid being rapidly effected. Therelore the arrangemellt of Figure 24 is extremely advantageous in both the heat effeciency and the stability of operation.
Also glow discharge heating apparatus such as s}lown in Figure 24 can be utilized to instantaneously heat a liquid, for example, water by flowing the water in a flow .
ratc of from 1 to 10 litres per minute ~hrough the interior of the electrodes thereby to transfer thermal energy injected into the electrodes to the water. Under these circumstances, water at room temperature must be heated to a temperature of about 80C. This results in the necessity of injecting thermal energy of at least 5 kilowatts into the electrodes. This means that, with a power source of AC 200 volts used, the effective current of at least 25 ampcres must rlow through the clectrodes. If a d;scharge current becomes high and also if the discharge gap is filled with a gas under an increasing pressure then it is difficult to sustain the flow dischargc. For example, the glow dis-charge transits to an arc discharge.
It has been found that the stable maintenance of the glow discharge is affected by the type of gas filling the discharge space. ~lso it has been experimentally con~;rmed that, by filling the discharge space with a mixture of at least helium and hydrogen, the glow disch-lrge can be sustained without the transit to an arc discharge, even with an electric power required for heating the p~rticular liquid, that is to say, a discharge current as high as ~ossible.
This will llOW be described in conjunction with Figure 24. Various experiments were been conducted with the discharge space 81 filled Wit]l an inert gas heavier than argon under a pressure ranging from S0 to 200 Torrs. The result of experiments indicates that the glow discharge is difficult to spread and that an increase in glow current causes a contraction of a positive column included in the glow discharge to move the glow discharge about on tlle electrodes 1 and 2. Thus the glow discharge is put in its unstable state so that it is apt to transit to an arc discharge. The mean value of the glow current in excess of

- 5~ -.;

,11 - 1~L'3~32¢.5 5 ampercs has caused the glow discharge to transit to an arc clischarge.
¦ l~ith neon u~ed, relatively stable glow discllarge ¦ has occurred under a gas pressure not higher than 70 Torrs.
¦ Under a gas pressure of 100 Torrs, however, the glow discharge ~;
¦ has been relativily stable at the deerctive current up to ¦ about 20 amperes. Upon the effective current exceediilg 20 ¦ ampere, the positive column has been contracted. This ¦ might cause the glow discharge to transit to an arc discllarge.
¦ Further, when an inert gas used has been heavier ¦ thall neorl, the ~catter rrom the clectrodes l and 2 has ¦ increased in amounts with the result that the electrodes ¦ l and 2 are violently consumed while insulating materials ¦ such as glass forming the enclosure 9 is sharply deteriorated ¦ in electrical insulation because metallic materials scattered ¦ from the electrodes 1 and 2 are sticked thereto. As a result, the useful life of the glow discllarge heating appara-tus has been inuch reduced.
I:rom thc .fOle~Oill~ :it is sllmmerizcd that, with the arrangement of Figure 24 used as a heating appara~us for instantaneously heating water, it is required to sustain stably the glow discharge under a relatively high pressure of 50 Torrs or more and still at a high currellt exceding 25 amperes at an AC voltage of 200 volts.
Also from the foregoing it has been founcl that it is desirable to fill the discharge space 81 wit~ a chemically stable, light inert gas and suitable examples of the inert gas involve helium and hydrogen.

2~5 In the arrangement of Figure 24, however, it has been seen that, with helium filling the discharge space 81, ;~
the flow discharge sprea~s throughout the surface of the electrodes 1 and 2 at low currcnt because of a small current S density and that electrical energy of the glow discharge entering the elec~rodes l and 2 amounts only to about 2 Killowatts. Also in a glow discharge caused in an atmosphere of helium, its positive cloumn llas been con~racted upon a pressure of helium increasing to 150 Torrs to increase a current density for the glow discharge. Thus the glow discharge has been moved about on the electrodes and become unstable. rhe glow dischar~e has oftcn transit to an arc discllarge.
On the other hand, a glow discharge in an atmosphere I5 of hydrogen has made a discharge hold minumum voltage Vo equal to at least 240 volts as shown in Figures 30, 31 and 32 which will be described hereinafter. Therefore, it has been difficult to cause a glow discharge having an electric ;
power of 5 kilo~atts or more by using an AC source with 200 volts.
It has been found that, in order to manufacture glow discharge heating apparatus requiring at least 5 kilowatts with an AC voltage of 200 volts, it is optimum to employ a mixture of helium (lle) and hydrogen (~12) as a filling gas.
When the arrangement of Figure 24 is filled with a mixture of helium and hydrogen under a pressure of 100 ;
Torrs, and applied with an AC voltage of 60 hertzs having a waveform E shown in Figure 26, a glow current flowing ~;
.
. ; ~
;; - 56 - ~
' ' ~ ~
.. .

~ ~
`

c ~ ~
~
therethrough is changed in accordance with a proportion of hydrogen to helium as shown at current waveforms F, G, H .
and I in Figure 26. Figure 26 shows the voltage and current waveforms in one cycle of thc source voltage. The current waveforms F, G, ll and I have been plotted with gaseous ~ :
mixture including 5, 10 30 and S0 % by volume of hYdrogen and the balance, helium respectively.
Also the glow discharge exhibits the current-to-voltage characteristic dependellt upon the pro~ortion of the hydrogen to the helium as shown in Figure 27 wherein a voltage in volts is plotted in orclinate against a current I
in amperes in abscissa and like reference characters have been employed to identify the helium-llydroge mixtures identical to tllose designated in ~igure 26. As shown in Figure 27, each of the current-to-voltage characteristics is substantially rec~ilinear. By calculating both values of glow voltages S, 1`, U and W through the extrapolation and slopes of respective characteristic curves, the glow voltage Vg may be approximately expressed by Vg = VO ~ RI
. ,' where VO designates a glow discharge hold minimum voltage ;-designated by S9 T, U or ~, and R designates the slope of ¦ the characteristic called the positive characteristic R.
As well known, the voltage VO is expressed by VO = Em sin ~t I where Em designates the peak value thereof and ~ designates ¦ an angular frequcncy of the source voltage. To calculate a . 1 ~ ~ ::

discharge power P form the above expression for VO referring to Figure 26 gives .

S TR ¦ (Em sin w~ - Vo) Em sin wt dt 2Em 1,~. Em . -lVo 1~R { 4 rl,n1 ~ W~ sln e Vo cos~sin E-m) }

where T designates a period of the source voltage~ The discharge voltage is thermal energy entering the electrodes :~
1 and 2 due to the glow discharge.
Assuming that the source voltage has its fre~uency ~:
of 60 hertzs and 200 volts or the peak value of .
Em = i~ 200 ~ 280 volts, its period is of 16.67 milli- :~
seconds and its angular frequency is of 377 radius per second. By using those figures in the expression for the discharge power~ the glow discharge hold minimum vcltage VO relates to the posi.tive characteristic R as shown in . .
Figure 28 wherein the positive resistance R in ohms is plot~ed in ordinate against the glow hold minimum voltage VO in volts in abscissa with the parameter being the r discharge power or thermal cnergy P.

:

q5 From the ~igure 28 it is seen that, in order to provide the thelmal cnergy not less tharl S kilowatts, the VO and R may lie in a hatched region as shown in Figure 28 defined by a line for tl-c power of 5 kilowatts, and both coordinate axes.
~lso the glo~ hold minimum voltage VO is determined by a pressure of ~ filli.ng gas and the gap length d .~ between the electrodes 1 and 2 while the positive character-istic R is determined by the configuration of the electrodes of the overlapping area SO for both electrodes 1 and 2 and the pressure o-f the filling gas.
By cllangillg a rel~tive di.clnleter M of one to the other of the clectrodes 1 and 2 to vary the ovcrlapping area SO therefor and also by changir.g the pressure of the lS filling gas, the positive characteristic R is varied as shown in Figure 29 ~herein the overlapping area SO in square centimeters is plotted in ordinate against the pressure of the filling gas in rorrs in abscissa with the positive characteristic R variously changed. In Figure 29 solid line indicates measured values and dotted line indicates values estimated from the associated measured values.
; From Figure 2g it is seen that9 under a gas pressure less than 50 Tolrs, a current density for the glo~ discharge ~.
is low and the supply of a discharge power or a heat input in excess of 5 kilowatts to the electrodes requires an increase in overlapping area S. I`his has encountered the problem in the portability because the electrode area must increases.

- 5g -.

~ ~: : ~

~$`$~i On the othcr hand, a gas pressure in excess of 150 Tolrs causcs tl-e dischalge inl7ut to the electrodes to increase to at least 5 kilowatts, rcsulting in a glow current of at least 25 amperes. lJnder these circumstances a positive column involved is contracted and the particular glow discharge is moved ahout Oll the electrodes rhis mi~ht sometimes cause the glow discharge to transit to an arc discharge.
With the gas prcssure furtller increased to 200 Torrs ]0 or higller, a posit;ve column involved is contracted at a glow curlent of at least 5 amperes until the transit to an arc discharge occurs.
~s an exampIe, it is assumed that tlle glol~ hold minimum voltage VO is impossible to decrease to 176 volts or less Under the assumed condition, it is seen from Figure 28 that, in order to manufacture glow discharge heating appal-atus having a discharge input of at least 5 Xilowatts, the prcssure of the particular filling gas, the OVCrl;117~ II`C,I .~o alld the positive cl~ar~cteristic l~
must lie in the hatched portion sho~n in Figure 29 as being defined by a pair of vertical broken lines passing through the abscissas of 50 and 150 I`orrs resl7ectively and curve labelled l~ = 2Q
In additio3l, by changing both the proportion of hydrogen to helium and the gap length d between the electrodes 1 and 2, the glo~ hol-1 mill;mum voltage V is varied as shown in ~igures 30, 31 and 32 ~herein the axis of ordinates rel7resellts the proportioll of hydro~en to helium in percent l~L''`~Z~5 and the axis of abscissas represents the gap length d in millimeters. Ihe helillm-]?ydro~ell mixture is mail1tained under pressures of 50, 10n and 150 Torrc; in ligures ~0, 31 and 32 rcspcctively. ~n thcse l:ig~Jres curvcs are labclled measured vallles of the glow hold minimum voltage V~ and for pure hydrogen the measured voltages VO are denoted aside corresponding dots.
Also the gap length c1 less than about 0.5 millimeter between both electrodes 1 and ~2) has resulted in a danger that both e]ectrodes may contact and sllortcircuit each other due to a pressure (li~ferencc betweelI a pressure of the particular heatecl liquid within either of the electrodes and that of a filling gas involved. On the other hand, an excessively large ~ap lengt]l d between both electrodes cause a positive column to constract to move the resulting discharge about Gn the electrodes until the discharge sometimes transits to an arc ~ischarge. This might result ; in a cause for damaging the electrodes l and 2. It has been seen that the contraction of the positive co]umn occurs with the gap length d of at least l~, 6 and 3 millimeters under the gas pressures of 50, 100 and 150 Torrs respectively.
With the proportion of hydrogen to helium decreased to 2.5% or less, the resulting glow discharge resembles ~ r that occurring in an atmosphere of pure helium. This has made it difficult to increase the discharge input to at least 5 kilowatts. Also as Figure 29 describes that it is difficult to decrease the positive characteristic R to at most 1, 0.5 and 0.3 ohms under gas pressures of 50, l00 and 150 Torrs respectively~ it llas been d~rficult to increase the discharge input to at least 5 kilo~atts at the glow hold minimum voltages VO of .It Ielst 2]0 2.~0 and 24n volts under the gas pressures of S0 100 al-d 150 rorrs respectively as will readily be understood rrom the graph shown in Figure 28.
Further an increase in glow hold minimum voltage V causes an increase in peak value of the glow current as shown in Figure 33 whereir the peak current for the glow discharge in amperes is plotted in ordinate against the ~low ho1d minimum voltage VO in volts in abscissa. I`his has resulted in the disldv;llltclge that the rc~ultillg al~lla-l.ltus slloll1d be mllde larger.
~rom tlle foregoing it will readily be understood that the proportion of hydrogell to helium and gap length d between the electrodes 1 ~nd 2 are desirably located in dotted closed areas shown in Figure 30 31 and 32. More specifically the ~roportion of hydrogen i.s not less than 2.5% and the ga~ lengtll d is not less than 0.5 millimeter while the voltage VO has values of 210 2~0 ancl 240 volts dependent upon tlle lressure of the ~illing gas.
While the present invention has been described in conjunction ~ith an AC source }-avillg a voltage of 200 volts it is to be understood that i~ is e~ually applicable to AC .
sources having the voltage higher than that of 200 volts for example the voltage of 400 volts. In the latter case the glow current may be low by using a helium-]lydrogen mixture including not less ~han 50~ by volume of hydrogen which is ef~ective for increasing the glow hold minim-lm voltage VO

~ 2~

shown at any o the points S, T, U and W illustrated in Figure 27. 'I`his provides a stable glow discharge while being able to decrease the surface area of the electrodes 1 and 2. In addition, wiring leads may be fine. l'herefore the rcsulting ap~aratus can be made compact.
Examples of the electrode material may involve copper, aluminum, nickel, ~ure ion, molybdenum, stainless steel, Kovar (Trade mark) etc. used with vacuum tubes or voltage regulator tubes. I-lowever, copper is not suitable for use in the present invention because the copper has a high current density for the glow ~ischarge to enhance the sputtering thereby to de-teriorate seversely the insulation of associated insulators. Also aluminum is not suitable for used in the present invention because a glow discharge involved transits to an arc discharge with a current as low as one ampere. Therefore suitable examples of the electrode material involve nickel, pure iron, molybdenum, stain]ess steel and Kovar ~Trade mark). 'l'he electrode used with tlle present invention has been formed of sheet nickel or stainless steel one millimeter thick.
~rom the foregoing it is seen that the filling of the discharge space 8 with a mixture including at least helium and hydrogen can eliminate -che transit of tJle glow to an arc discharge and the sputtering with a high discharge current. 'I`his gives the result that a stable glow discharge can be sustained. The reason for wnich the glow discharge can be prevented from transiting to an arc discharge is to remove oxides on ~]~e surface of the electrodes by the hydrogen 1Z~5 included in the filling gaseous mixture.
The use of the helium-hydrogen mixture is also advantageous in that, only by changing the proportion of the hydrogen to the helium, the glow hold minimum voltage can be selected at will to control the discharge input to both electrodes involve~ as d~sired.
Figure 34 shows still another modification of the - present invention. The arrangement illustrated is differenk from that shown in Figure 24 only in that in Figure 34 the opposite surfaces of the electrodes 1 and 2 are corrugated to increase the surface areas of the electrodes and an auxiliary electrode 46 is operatively associated with the ~ap 8 formed between the c]ectrodes 1 and 2 as will be subsequently described.
lS In glow discharge heating apparatus having the discharge input of 5 kilowatts, for example, the diameter M of the electrodes 1 and Z is re~uired to be of at least 80 millimeters and also that o-f the insulating enclosure 9 is necessarily of at least lob millimeters. In other words, tlle larger the diameter of the electrodes the larger khe enclosure 9 and therefore the seal fit~ings 10 and 11 will be. This is attendcd with the disadvantages that the compo-nents become excessively expensive and also a manufacturing .~.
cost is increased.
In addition, the opposite surfaces of the electrodes 1 and 2 are can be forced toward each other to be crowned in response to a di-fference between ~ pressure within discharge space 81 and a pressure of a heated liquid within az~

each electrode so that the bending of the electrodes increases to be proportional to the fourtll power of the radium M/2 thereof. Accordingly, an increase in diameter of the :
electrodes may causes the clectrodes 1 and 2 to contact and short circuit each other due to the crowning thereof.
To avoid this objection, the oposite surfaces of the electrodes 1 and 2 have a diametric section of corrugated shape to increase areas of the opposite electrode surfaces with the diameter of the electrodes remaining unchanged.
In the arrangerncnt of ~igure 34 each electrode 1 or 2 has ~;
tlle diameter M of 52 rnillimeters and the area of 80 sc~uare centimeters of tllat surface thereof opposite to the ot~ler electrode 2 or 1.
As shown in Figure 34, the auxiliary electrode 46 is extended and sealed through the insulating enclosure 9 so as to center the gap 8 formed between the opposite corrugated ~:
surfaces of the electrodes 1 and 2 and to be substantially contacted at the free end by the adjacent portion of the edge o~ the gap 8.
Then the Ac source 31 is connected at one end to the electrode terminal 5 through a normally open switch 45 and at the other end di.rectly to the electrode terminal 6. The auxiliary electrode 46 is connected to the electrode terminals

6 and S through respective resistors 47 and 48 and also by :.
a resistor 49 to one output o~ an auxiliary source circuit 50.
The auxiliary source circuit 50 includes the other output connected to the electrode terminal 5 and therefore the switch 45 and is also connected to the switch 45 through B-~5 ¦ ~

another normally open switch 51 and to t~ ~ other end of the AC source 31. I`he operation of the abovementioned circuit configuration will be described hereinafter.
With the auxiliary electrode 46 operatively .
associated with the discharge gap 8 as in the arrangement of Figure 34 the electrodes 1 and 2 are called hereinafter the "main electrodes" to be distinguished from the auxiliary electrode 46.
In the arrangement of Figure 34 a glow clischarge is fired between the mairl electrodes 1 and 2 after wilich the glow discharge is smoothly spread on the corrugated surfaces la and lb respectively of the main electrodes 1 and 2. Under these circumstances a high current can enter the opposite corrugated surfaces of the main electrodes 1 and 2 as compared with pairs of discharge electrodes including the opposite flat surfaces. Therefore the discharge input to the electrodes increased while the voltage across the main electrodes remains unchanged.
As a result the corrugated surface of the main i`
electrodes permits a decrease in diameter thereof attended -with a reduction in diameter of each of the insulating ~'~
enclosure 9 and the seal fittings 10 and 11. Accordingly a manufacturing cost can be decreased. Also the corrugated surface of the main electrode is effectiv'e for preventing the crowning of the opposite surfaces thereof.
'I`he opposite surface la of the main electrode 1 shown in Figure 35 includes a plurality of grooves of rectangular cross section concentrically disposed at substan-- 66 - ' `

~ 3~ 15 ¦ tially equal intervals thereon.
¦ Figure 36 shows a plurality of parallel grooves ¦ disposed at predetermined intervals on ~he discharge surface ¦ la of the main electrode 1.
¦ The discharge surface la of the main electrode 1 ¦ shown in Figure 37 includes a plurality of cylindrical ¦ depressions disposed in a predetermined pa~tern thereon.
In the arrangement shown in Figure 38, a pair of flow confining blocl~s generally designated by the reference numeral 200 and 210 respectively are of the same counstance-tion and disposed in place within tlle main electrodes 2 and 1 to form heating spaces or flow paths 2A and lA for a ;
heated liquid therein respectively. The flow confin;ng block 200 is formed of an electrically insulating material such as a synthetic resinous material and includes a feed water tube 201 and a drain tube 2n2 formed in parallel relationship on the exposed end sur~ace thereof to be integral therewith and through openings 201a and 202a connected to the tubes 201 and 202 respectively. Then openings 201a and 202_ open on that end surface thereof facing the inside o-f the gap forming surface of the main electrode 2 and a peripheral surface thereof respectively.
The tube 201 and the opening 20a interconnected serves as a feed water tube opening in the flow path 2A while the tube ~;
202 and the opening 202a interconnected serves as a drain tube also opening in the flow path 2A.
The flow confining block 210 includes a feed water and a drain tube identical to those as above described in :

:~

conjunction with the flow confiring block 200 and designated by like reference numeral identifying the corresponding components of the confining block 200 and added with the . .
numeral lO. For example, the reference numeral 211 designates - :
a feed water tube. .
~he flow confining blocXs 200 and 210 have the exposed end portions screw threaded through the blind cover plate 22 and 23 fixed to the open end portions of the main electrodes 2 and 1 to be flush with the open ends thereof ~:
respecti.vely.
In other respects, the arrangement is substantially identical to that shown in Figure 34 except for the omission of the insulating tubes 3?. 38, 39 and 40 shown in Figure 34.
In the arrangement of Figure 38, the flow confining blocks 200 and 210 can be removed from the blind cover :~
plates 22 and 23 respectively for the purpose of inspecting or cleaning the internal surfaces of the main electrodes 2 and 1. Therefore the hea~ing efficiency can be always maintained high.
Figure 39 shows modification of the arrangement shown in Figure 15 wherein the user is accessible to the heat transfer surfaces of the main electrodes as in the arrangement of Figure 38 and an auxiliary electrode 46 is operatively associated with the discharge gap 8. As shown in Figure 39 a flow confining tube 200 in the form of a hollow cylinder havin~ both ends open is coaxially disposed witllin the main electrode l to form a flow path for a heated liquid therebetween. The cylindrical tube Z00 is screw `~ ~

¦ threadcd througll a screw mernber 200a rigidly fitted into the ~; ¦ open el~d of the main electrocle 1.
¦ Similarly allothe-r f1OW confirling tube 210 in the . ¦ form of a hollo~ cylinder hav;ng one end closed is detachably :. 5 ¦ connected to the maill electrode 2 at the outwardly folded ¦ end through a screw member 210a formed internally with the tube 2.10 to form an annular flow path for the hea~ed liquid therebetween. .~:
The flow confining tubes 200 and 210 are of an electrically i.nsulating material such as a synthetic resinous material.
As in the aTrangement of Figure 38, the flow confining b~ocks 200 and 210 can reaidly be removed from the main electrodes 1 and 2 respectively for purposes of inspection and cleaning. ~ :
In other respects, the arrangement is substantially similar to that shown in Figure 15 excepting that electric shock preventi.ng means such as above described in conjunc-tion with Figure 24 are provided on the feed water and drain tubes 41, 42 and 43, 44 and the auxi.liary electrode 46 is operatively coupled.to the gap ~ formed between the main .
opposite electrodes 1 and 2.
Figure 40 shows a different modification of the ... ~.
present invention ena~led to decrease the dimension of the electrically insulating enclosure and still increase the diameter of the main electrodes. In the arrangement illustrated a pair of main electrodes 1 and 2 identical to each other are horizontally disposed in opposite relationship ,ll ; .. ;

to form a discharge gap 8 therebetween. Each of the main ~-electrodes 1 or 2 is in the form of a hollow cylinder having one end closed and the other end portion lB ~r 2B reduced ~:
in diameter. The closed flat cnds of both main electrodes 1 and 2 form therebetween the ~ap 8 having a width or a gap length of d and a diameter of M.
Each electrode 1 or 2 includes a shoulder connected to an electrically insulating enclosure 9a or 9_ in the form of a narrow annulus through a first annular sea] fitting lOa or 11_. Thus the enclosures 9a or 9_ encirc]es the reduced diameter cnd portion lB or 2B of thc main electrode 1 or 2.
~hen a cylindrical metalli.c shell 9c or 9d encircles in spaced relationsnip the adjacen-t main electrode 1 or 2 and includes a radially inward directed ~lange connected at one lS end to the enclosure ga or 9b through a second annular seal fitting 10 or llb. Both shells ~c and 9d have the other end.s abutt;ng against and fixed together as by welding.
Thus the shells 9c and 9d and the lnain electrodes 1 and 2 form therebetween an annular discharge space 81 including the gap 8 with the encl.osures 9a and 9_ the seal fittings lOa, lOb 11 and llb.
The lind cover ~late 22 or 23 is rigidly fitted into the open end of the main electrode 1 or 2. A feed ..
water tube 41 or 42 is extended and sealed through the blind cover plate Z2 or 23 and has an outlet opening substantially flush with the internal surface of the cover plate 22 or 23. Also a drain tube 43 or 44 is extended and sealed ough the blind cover plete 22 or Z3 and llas an end .~

portion bcnt into an L in order to fill a hcating space lA
or 2A formed of the interîor of the main electrode 1 or 2 with a li~uid to be heated. The end of the L-shape tube 43 or 44 faces the u~ ermost portion o-E the internal surface S of the main electrode 1 or 2 with a distance QO maintained therebetween.
Furt]ler, the auxiliary electrocle 46 and an associated electric circuit are provided in the same manner as above described in conjunction with Figure 34.
The main electrodes 1 and 2 may be of any desired shape other than the cylin(3rical shape as above descrihed.
As the main electrodes 1 all~ 2 are of the same structure, thc Opel'.ltiO]l w;ll now bc descrihc(l ;n conjunction of one of the electrodes, for example, the electrode 1.
A liquid to be heated entcrs the heating space lA
throu~h the feed water tube 42 as s]lown at the arrow A in Figure 40 until its liquid surface reaches a level at which the drain tube 44 opens while the liquid is heated by the main electrode 1. ~her2after the heated liquid is exhausted from the space lA througIl the drain tube 44 as shown at the arrow B in Figure 4Q. I`he outflow of the liquid causes a pressure loss across the drain tube 44 permitting the heated liquid charged in the hcating space lA to have a pressure higher than the atmospheric pressure. In keeping with this increase in pressure, the surEace o-f the liquid within the heating space lA is forced to be gradually raised beyond the open end of the drain tube 44 resultin~
a decrease in volume of a cavity existing in the heatillg _ 11 r ' . :'' , ~$~ :
.
space lA.
In this case, the smaller the diameter of the drain tube 44 will be which is accornpanied by an increase in speed of the liqui.d flowing through the drain tube 44. As ~, a result, the open end of the drain tube 44 is less in pressure than the cavity within the heating space lA. This causes an increase in rate at which the drain tube 44 sucks up air left within the heating space lA. ::
It has been experimentally proved that the distance :~
: 10 QO exceeding 10 millimeters causes the air phase in the heatin~ s~ace lA to be too ~ar spaced from that ~ortion of the liquid just flowing through the open end of the drain tube 44. 'I'herefore the heating space lA has been difficult to be sufficultly deaerated. This means that the distance QO is preferably of at most 10 millimeters.
_ In othcr words, the distance Q is so dimensioned that, even though steam bubbles would be evolved from the :~
liquid being heated within eithcr o~ t.lle heatin,~ spaces lA
and 2A and re.lch the uppermost portion of thereof, they can be rapidly exhausted throu~h the drain tube 43 or 44.
After the air has been fully removed frnm either of the he.ating spaces lA and 2A as above desdribed, both spaces is entirely filled wi.th the heated liquid without the steam bubbles accumulated to form a cavity therein.
Otllerwise a cavity not filled with the hcated liquid is ..
formed within eit]ler o~ ~he main electrodes 1 and 2 and tl?erefore that portion thereof contacted by ar.d loca~ed adjacent the ca,ity excess;vely rises in temperature .i resulting in its ailure.
¦ The arrangement of Figure 40 is further advantageous in that the insultaing enclosures decrease in diameter and ¦ therefore are easily manufactured with low cost and S ¦ mechanically strong because the enclosures surround the ¦ reduced diameter portions of the main electrodes which are ¦ encircled by the metallic shells interconnected into a ~ ;
¦ unitary structure to permit a region occupied by the ¦ insulating enclosures to be extremely decreased. Further ¦ the main electrodes are insulated from the shells through ¦ the insulating enclosures respectively. Accordingly, the ¦ resulting apparatus is easy to be manufactured, inexpensive ¦ and robust while having a long useful life. ~' In the arrangement shown in Figure 41, the insulating enclosure 9 in t~le form of a hollow cylinder having both ends open includes a pair of upper and lower apertured -~
cover plates 13 and 14 respectively connected to both open ends thereof through annular seal fittings 10 and 11 respectively. A pair of hollow main electrodes 1 and 2 having one end open are vertically disposed in opposite parallel relationship within the enclosure 9 to be staggered longitudinally of the enclosure and form a discharge gap 8 in a discharge space 81 defined by the enclosure 9, the seal fittings 10, 11 and the cover plates 13 and 1~. The main electrodes 1 and 2 have the other open ends fixedly fitted into apertures on the upper and lower cover p a~es 13 and 14 to be flush with the outer surfaces thereof respectively.

Zq~

The rnain electrodes 1 and 2 have the open cnds closed with blind cover plate 23 and 22 having central openings respectively. Tllen a L-shaped tube 44 or 41 has one leg connected to the ccntral opcning on the blind cover plate 23 or 22 and the otller leg horizontally extended to form an outflow or an inflow tube.
A feed water tube 42 extends in sealing relationship through the one leg Or the outflow tube 44 and into a heating space lA within the main electrode 1 Ero~ above the upper plate 13. Similarly, the drain tube 43 extends through the inflow tube 14 and into a heatillg space 2A within the main electrode 2 from below the lower plate 14.
As in the arrangement o~ Figure 40, the drain tube 43 has its open end facing the inside of the closed end of the main electrode 2 through a spacing QO not greater than 10 millimeters.
As sh~on in Figure 41~ the inflow tube 41 has the end opening in the heating space 2A below the inlet of the drain tube 4~ while the fced liquid tube 42 has the end opening in the heating space lA below the inlet of the drain tube 44. Therefore the heating s~aces lA and 2A can be entirely filled with the heated li~uid as in the arrangement of Figure 40.
Further an auxiliary electrode 46 is operatively associated with the discharge gap 8 formed between the main opposite electrodes 1 and 2. If desired, both main electrodes may be concentrically disposed.
In the arrangement shown in Figure 42, a seamless `:

metallic tube is closely wound into a helix 41a or 42a having the outside diameter substantially equal to the inside diameter of the main electrode 2 or 1. 'I`he helix 41a or 42 includes one end portion 43 or 42 extending through the central hollow portion thereof and the other end portion 41 or 44 bent into an L-shape. Both helices 41a and 42a are inserted into the main electrodes 2 and 1 to be brazed or welded to the intcrnal surfaces thereot respectively for the purpose of improving the heat transfer Erom the mating main electrodes thereto. A liquid to be heated enters the helix 41a or 42a througll the end portion 41 or 42 and leaves the end portion 43 or 44.
In other respects, the arrangement is identical to that shown in Figure 41.
Each of the main electrodes 1 or 2 can be prevented ~:
from corroding starting with those portions thereof brazed or welded to the helix 42a or 41a because the brazed or welded portions are not directly contacted by the heated liquid flo~ing through the helix. .Since the heated liquid flows at a high speed through the helix 41a or 42a, the nuclear ebullition can be prevented and also a pressure loss in the helix is increased to prevent steam bubbles from staying in the helix. l`his results in the smooth heat transfer from the main electrode to the heated li~luid flowing through thc m~ting helix. IhUS the main electrodes are prevented from excessively rising in surface tem]-erature thereby to sustain stably the glow discharge.
The arrangement sho~n in ~igure 43 is substantially .,,.1~
. ~

similar to that illustrated in Figure 40 exeep-ting that, in addition to dispos:ing the main electrodes 1 and 2 vertically, they are in the form of square hollow prisms and a tube is closely wound in helix complementary in shape to the interior of the associated main electrode and fixed thereto.
Each of the arrangements shown in Pigures 42 and 43 is characterized in that tube means formed of a good thermally conductive material contacts the internal surface of the mating main electrode to be thermally integral therewith and the heated liquid flows through the tube means. This results in thc alleviation o~ limitations as to the configura-tion of the main electrode while facilitating the manufacturing of the apparatus and prolonging the useful life.
In the arrangement shown in Figure 44 either of the blind cover plates 22 and 23 is provide~ on that portion diametrically opposite to the nor~al outlet with an exhaust port that is, in turn, closed with a plug 221 or 231 for example throu~h a screw ~neahcnism. Further an auxiliary electrode 46 is operatively couple~ to the gap formed between the main opposite electrodes 1 and 2 as above described in conjunction with Figure 34.
In other respects, the arrangement is substantially identical to that shown in Figure 24.
The arrangement shown in Figure 45 includes the U-shaped flow path ~r heating space lA or 2A within the main .
electrode 1 or 2 and a connecting tube 361 or 351 connected to the heating space ~A or lB on the inlet side. Then the connecting tube 361 or 351 is provided with an exhaust ~ : ~

port closed with a detachable plug 231 or 221.
In other respects the arrangement is substantially identical to that shown in ~igure 44.
When each of the arrangeTncnts shown in Figures 44 and 45 is desired to be out of service Eor a long time, the plugs 221 and 231 can be removed from the associated exhaust ports to drain the liquid out rom interior of the main electrodes for the purpose of preventing the liquid within the main electrode from spoiling or freezing. Also the useful life can be prolonged.
While the main electrodes have been described as being in the form of hollow cylinclers having the same shape and disposed in opposite relationship it is to be understood that the main electrode may be of any other desired shape.
For example, the main electrodes may be in the form of hcllow cylinders disposed in coaxial relationship. It is essential -~
that, in order ~o empty the interior of the main electrodes, the exhaust port must be provided on the lower portions ; thereof.
While some of the abovementioned Figures~ for example~ Figure ~4 illustrate the control circuit for controlling the ~low discharges Figure 46 shows the fundamental circuit configuration of a control circuit for controlling any of the arrangements as above described including no auxiliary electrode. In Figure 46, the arrangement generally -~
designated by the reference numeral 100 comprise~ a pair of first and second electrodes l and ~ respectively disposed in opposite relationship to form therebetween a gap having a --~ ,li q~

¦ gap length or a width d and each including an inflow and ~`
an outflow tube. Water enters the interior of either electrodes l and 2 through the inflow tube to be heated I and heated water lcaves it thro-lgh tl~e outflow tube.
¦ The source of A~ voltage 31 is connected across the electrodes 1 and 2 through a bidirectional triode thyristor 60 with the first electrode l connected to ground. The ¦ bidirectional triode thyristor is called hereinafter a l "Triac" (grade mark). The source ~n is also connected ¦ across a gate circuit 61 through a normally open switch 62.
¦ Then the gate circu;t is connected across one electrode and ¦ a gate electrocle of the Triac 60. The switch 62 is closed ¦ to fire a glow discl-large between the electrodes l and 2 there ¦ by to heat a liquid, for example, water flow;ng through the ¦ interior of each electrode.
¦ The operation of the control circuit shown in ¦ Figure 46 will now be described with reference to Figure 47 ¦ wherein there are illustrated a voltage waveform V supplied ¦ from the source 31 and having a peak value Em and a current ¦ waveform V of the glol~ discharge. As shown in Figure 47, ¦ the voltage waveform V in the ~ositive half-cycle of the ¦ source gradually increases from its null point until time ¦ point tl is reached. At that time voltage reaches a value ¦ of a discharge br~akdown voltage V5 to fire a glow discharge ¦ between the electrodes l and 2. At that time pOi]lt tl a ¦ glow current I abruptly flows through the electrodes 1 and ¦ 2. The glow current I corresponds to a voltage drop expressed ¦ by Vf - VO where VO designates a glow hold minimum voltage , ,1 ;~

8Z~i5 ¦ and may be expressed by I = (Vf - Vo)/R where R designates a discharge resistance corresponding to a slope of a I current-to-voltage characteristic curve for a glow discharge ¦ as above described in conjunction with Figure 8.
¦ The at time point t2 the voltage Y is equal to the ¦ glow hold minimum voltage VO after which the glow discharge ¦ is extinguished because the voltage is less than the I voltage VO.
¦ Thereafter the sourc3 31 enters the next succeeding ¦ negative half-cycle of the source in which the process as above described is repeated to cause a glow discharge between the electrodes 1 and 2. In the arrangement shcwn in Figure 46 the application of the AC voltage causes the electrodes l l and 2 to act alternately as a cathode and an anode electrode ¦ respectlvely to be heated because the glow discharge heats that electrode acting as the cathode as above described.
From the foregoing it will ~e seen that, the firing of the glow discharge at time point tl causes an instantaneous increase in glow current so that the glow discharge can not spread following this increase in glow current. This results in the tendency to locally concentrate the glow current on the electrode to transit the glow discharge to an arc discharge. The arc charge has a fear that it melts the electrode which~ in turn, reduces the useful life of the heating apparatus.
Also the glow current is initiated to flow through the electrodes l and 2 only upon the source voltage across both electrodes reaching the discharge breakdown voltage V~

. ~,,. : . . . , , . .. ; . , : . .

3Zg~S

while Vf ~ VO hilds. Therefore it is impossible to utilize a time interval during wh;ch the source voltage is not less than the glow hold minimum voltages VO as a coDduction time resulting in a poor efficiency o~ utilization of the source S ~oltage.
Figure 48 shol~s a control circui~ for controlling the glow discharge heating apparatus of the present invention constructed in accordance with the principles thereof. The arrangement illustrated comprises an auxiliary source circuit 61 connected across the source of AC voltage 31 that supplies hC voltage of 200 volts at the commercial :Erequency. The circuit 61 includes a normally open switch 62, a step-up transfor~er 63 having a primary winding connected across the source 31 through the switch 62 and a secondary winding having one end connected to the electrode 2 through a current limiting resistol~ 64 and the other end connected to theelectrode 1 and also to ground.
As in the arrangement of ~igure ~6, the source 31 is connected to the electrode 2 ~hrough the Triac 60. The resistor 64 is connected across a primary winding of an electrically insulating transformer 65 including a secondary winding connected across a pair of AC inputs of a rectifier bridge 66. T~he rectifier bridge 66 include a pair of DC
outputs one of which is connected to the junction of the source 31 and the Triac 60 through a resistor 67 and the c,ther of which is connected to the remaining terminal or a gate terminal of the Triac 60.
The step-up transformer 63 is desigrled and ~\

constructed so that the discharge breakdown voltage Vf is applied across the electrodes 1 and 2 befor`e time point where an instantaneous voltage from the source 31 reaches the glow discharge minimum voltage VO.
The opera~ion of the arrangement shown in Flgure 48 will now be described with reference to Fi~ure ~9 similar to Figure ~7. In F;gure 49 wherein li]~e reference characters designates the components corresponding to those showll in Figure ~7, the switch 62 is closed at time point A to permit the source to apply the source voltage across ~he primary winding of the trans~ormer 63. At a point B, a secondary or an output voltage from the transformer 63 reaches the discharge breakdown voltage Vf whereupon the gap between the elec~rodes 1 and 2 is broken down to start an electric discharge therebetween. At that time the output voltage drops to the glow hold minimum voltage VO (see point C, ;
Figure 49) -for a glow discharge by means of the current limiting resistor 64. This causes a current i on the order of 0.1 ampere to flow through the electrodes 1 and 2 resulting in a glow discharge occurring across the electrodes 1 and 2.
That glow discharge is called a "pilot glow discharge".
The current for the pilot glow discharge causes a voltage drop across the current limiting resistor 64 that, in turn, induces a secondary voltage across the transformer 65. The secondary voltage from the transformer 65 is applied to the gate electrode of the Triac 60 after having been full-wave rectified by the rectifier Iridge 67 to put the Triac 60 in its conducting state. Therefore the source voltage is , ..
ll , applied across the electrodes 1 and 2. Under these circumstance, if the pilot glow discharge has not occurred across the electrodes 1 and 2 then the pilot glow current i does not flow through the electrodes 1 and 2 and no voltage is induced across the insulating transformer 65 with the ~esult that the Triac 60 is maintained non-conducting.
This ensures that the application o~ the high AC voltage across the electrodes 1 and 2 does not rcsults in the ;
occurrence of an arc discharge therebetween unless the pilot glow discharge preliminarily occur across the ;~
electrodes 1 and 2.
When the source voltage is applied across the electrodes 1 and 2 through the now conducting Triac 60 and reaches the glow hold minimum voltage VO' the principal glow discharge is fired across the electrodes 1 and 2.
That is, a current I for the principal glow discharge flows through the electrodes 1 and 2. That principal glow discharge current I is extinguished aFter the source voltage V has again reached the glow hold minimum voltage VO at point E
or time point t2 and therefore the principal glow discharge is extinguished. }-lowever it is noted that at point E the voltage VO from the step-up transformer 63 is applied across the electrodes 1 and 2 through the resistor 64 with the result that the pilot glow dlscharge is still established.
Then at point F, the output voltage from the step-up transformer 63 becomes also less than the voltage VO to cease the pilot glow discharge.
Then the source 31 enters the next succeeding negative s half-cycle in which the process as above described is repeated.
The concept of the embodiment of the present invention as shown in Figure 48 is to apply preliminarily a high voltage across the electrodes by means of the auxiliary S cource circuit to cause the preliminary or pilot glow discharge thereacross and to smoothly derive the principal glow discharge from the pilot glow discharge. Therefore the arrangement of Figure 48 is effective for preventing the principal glow discharge current from abruptly increasing resulting an arc discharge as in the arrangement of Figure 46. Further the efficiency of utilization of the source is increased.
The arrangement shcwn in Figure 50 comprises a reactor 68 connected between the source 31 and the electrode lS Z, and an AC pulse generator 69 connected across the source 31 through the normally open switch 62. The pulse generator ~9 includes one output connected by the current limiting resistor 64 to the junction of the reactor 68 and the electrode 2 and the other output connected to the electrode 1 and therefore to ground.
The gap formed between the electrodes 1 and 2 is so `
dimensioned that the peak voltage Em from the source 31 is prevented from effecting the discharge breakdown of the gap.
As shown in Figure 51 wherein a voltage and a current waveform V and I respectively and a pulse waveform P are illustrated, the AC pulse generator 69 generates an AC
pulse voltage P sufficient to reach the discharge breakdown voltage Vf at time point tl where the voltage from the ,,11 ~ ~

'~ 2 ~ ~

source 31 approximately reaches the glow hold minimum voltage VO. The pulse voltage P first effects the discharge break-down of the gap between the electrodes 1 and 2 followed by a flow of the principal glow current I th.rough the electrodes.
As in the arrangement of Figure 46, the current I
becomes null at time point t2to extinguish the glow discharge ~fter which the process as above described is repeated in the next succeeding negative half-cycle.
It is noted that the reactor 68 is dcsigned and constructed so that it present a high impedance to t~e pulse wave~orm P but a low impeclance to the commerclal frequency of the source 31.
Thus the arrangement of Figure 50 ensures that, ~ihen the source voltage V is close to the glow hold minimum -voltage VO~ the principal glow discharge is initiated between the electrodes 1 and 2 and then the principal glow currPnt I is smoothly increased without the transit to an arc discharge.
Figure 52 shows a modification of the present inventi.on wherein the ~ilot glow discharge occurs between the auxiliary electrode and either of the main electrodes prior to the principal glow discharge as above described, for example, in conjunction with Figure 34. In Figure 52, the main and auxiliary electrodes 1, 2 and 46 respectively are schematically shown and may have any of their structures shown in Figure 34 and Figures 38 through 45.
The arrangement illustrated comprises the AC source 31 and an auxiliary source shown as comprising a step-up ~ ~l ~
~ ~:
~ ~ ::
transformer 70 including a primary winding connected across the sourcs 31 through the normally open switch 51 and a c:enter-tapped secondary winding. The dot convention is used to identify the polarity of the instantaneous voltage across the associated winding. The secondary winding includes center tap connected to the auxiliary electrode 46 through a current lirniting resistor 71 and a normally open switch 72, and a pair of end terminals connected ~o the main electrodes 1 and 2 through individual semicondllctor rectifier diodes 73 and 74 with anode electrodes thereof connected to tlle main electrodes respectively. 'rhe gap formed between the electrodes 1 and 2 has a distance or a gap length d satisfying Vf > Em ~ VO' where Vf, Em and VO have been previously defined. ;
lS The switch 51 is closed to apply the AC voltage V
from the AC source 31 across the electrodes 1 and 2 while ;~.
the switch 72 is closed to apply a high voltage waveform fIom the step-up transformer 70 to the auxiliary electrode 46. Under these circumstances, when a potential at the main ~20 electTode 1 is higher than that at the main electrode 2, the diodes 73 and 74 are turned off and on respectively to cause a pilot glow discharge between the auxlliary electrode 46 actlng as an anode and the main electrode 2 acting as a cathode. On the contrary, when the main electrode 2 is higher in potential than the main electrode 1, the diodes 73 and 74 are turned on and off respectively to cause a pilot glow discharge between the auxiliary electrode 46 acting as anode and the main electrode 1 as the cathode.

- ::

In addition, as the auxiliary electrode 46 has applied thereto the voltage Erom the center tap on the secondary transformer 70 winding, the voltage applied across the auxiliary electr~de 46 and the main electrode 1 to cause the pilot glow discharge therebetween is quite identical to that applied across the auxiliary electrode 46 and main electrode 2 to cause the pilot discharge therebetween.
Therefore, the transit of the pilot glow discharge due to : the auxiliary electrode to the principal glow discharge between the main electrodes 1 and 2 are equally effected ~.
between each of the positive half-cycles and the negative half-cycle of the source 31.
Further the occurrence of the pilot glow discharge ~
completes a closed circuit including the diode 73 or 74, ~ ~-the associated half of the secondary transformer 70 winding, : the resistor 71, the closed switch 72 and the pilot glow . discharge between the auxiliary electrode 46 and the main : electrode 1 or 2. This prevents the current for the pilot glow discharge from entering a circuit with the source 31.
. Z The opening of the switch 72 ceases the pilot glow ::
discharge from occurring between the auxiliary electrode 46 :
and either of the main electrodes 1 and 2. Thus the prlncipal glow discharges are not fired in the next succeeding cycle of the source and the cycles following the la~ter with the result that the heating operation is not performed. In other words, the ON-OFF control of the principal glow discharge -:`
~ can be conducted by turning the pilot discharge on and off.
: It is noted that the pilot glow discharge always . ~, -- 86 - -.
.~
.~, ~ 32~
,. .
occurs between the auxiliary electrode 46 acting as the anode and either of the main electrodes 1 and 2 acting as :.
the cathode so that the auxiliary electrode 46 is no~. heated. .
This results in the elimination o the necessity of cooling the auxiliary electrode. .:~
From the foregoing it is seen that, the arrangemen~
when effecting the ON-OFF control of the heating apparatus proper of Figure 52 ensures the transit of the glow ~-:
discharge by turning the pilot glow discharge on and off.
~10 The arrangement illustrated in Figure 53 is different from that shown in ~igure 52 only in that in : Figure 53 a zero-voltage firing circuit is provided to : prevent the glow current from bruptly increasing. In Figure 53 a pair of serlally connected resistors 75 and 76 are connected across the AC source 31 through the normally open s~itch Sl to form a voltage divider, and the junction A of both resi.stor is connected to a resistor 77 subse~uently connected to a base resistor 78 that is connected to a base source VBB. Ihe resistor 76 is connected to ground. The :
junction B of the resistors 77 and 78 is connected to a base electrode of an NPN transistor 79 including an emitter ~ electrode connected to the resistor 76 and a collector ; electrode connected to a DC source Vcc through a collector : resistor 80. The transistor 79 has connected across tlleemitter and base electrodes a semiconductor diode 81 serving to prevent a high reverse voltage from being applied across those electrodes and also connected across the collector :~
and emitter electrodes a differentiating circuit including a . ~
~ 45 capacitor 82 and a resistor 83. The junction of that collector electrode and the capacitor is designated by the reference character C and the junction of the capacitor 82 and the resistor 78 is designated by the reference character ~ only for purposes of illustration.
The junction D is connected to one AC input to a :~
rectifier bridge 84 including the other AC input connected`
to the resistor 83. The rectifier bridge 84 incl~des a ~ `
~air of DC outputs connected across a resistor 85 ~hat is connected at one end to a gate electrode of a Triac 87 through a normally open switch 86 and at the other end to the primary winding of the transformer 70. The Triac 87 is connected across AC source through the primary transformer 70 winding and the switch Sl and has connected thereacross a series combination of a capacitor 88 and a resistor 89 serving as an absorber.
The components 75 through 89 as above described form a zero-voltage firing circuit generally designated by the reference numeral 90.
l~ith the switch 51 closed, an AC voltage developed :::
at the point A is similar to the source voltage and sinusoidal as shown at waveform A in Figure 54. The AC ,:
sinusoidal voltage passes through its zero voltage points ~ ~
at time points _O, tl and t2 in each cycle of the source 31. ~ ::
Assuming that the source VBB is at a null potential, a voltage developed at the point B is sinusoidal between time points to and tl or in the positive half-cycle of the source and Temains null between time point tl and t2 or in the 1 ~3 ~$2~L~
I ............................................................ ' ,:~
~: I . .::
negative half-cycle thereof by means of the action of the di.ode 81 as shown at waveform B in Figure 5~. Since the transistor 79 is turned on only in response to a voltage l applied to the base electrode to render the latter positi.ve ~ ;
¦ with respect to the emitter electrode, the same is in its :`~
ON state between time points to and tl and in its OFF ¦
state between time points tl and t2. Accordingly, a `
voltage developed at the point C is null when the transistor :.
79 is in its ON state and equal to a voltage across the source Vcc also designated by Vcc when it is in its OFF
state as shown at ~aveforrn C in ]igure 54. :
The voltage at the point C is differentiated by the differentiating circuit 82, 83 to produce alternately a negative and a positive pulse at the point D as shown at. ~:
lS waveform D in Figure 54. Those pulse are rectified by the rectifier bridge 84 to form positive pulses which appear :
at a point E connected to the swi.tch 84 at time points to~
tl and t2 as shown at waveform E on Figure 54.
With the switch 86 closed, the pulses shown at waveform E in Figure 54 are successively applied to the gate electrode of the Triac 87. In other words, gate pulses are necessarily developed at the gate electrode of the Triac 87 at the zero passage points of the source voltage or at time points to, tl and t2. Thus it is seen that, even though the switch 86 has been closed at any time point, the Triac 87 is brought into its ON state starting with the ~ .
zero passage point of the source voltage. As a result, a :~
pilot voltage from the transformer 70 is applied to the ''.- '`` .~,~Zf~.~i . l auxiliary electrode 46 starting with the zero passage point of the source voltage or time point to, _l or t2 with the result that the principal glow current is prevented from ¦ sharply increased. This means that a li(luid f]owing in ¦ heat transfer relationship along the internal surface of each electrode 1 or 2 is smoothly heated.
The arrangement of Figure 53 is advantageous in ¦ that a principal glow current is prevented from sharply l rising at a firing time point and thc glow discllarge is prevented from transiting to an arc discharge due to the local conccntratioll o~ the currellt while e~Çiciency oF utlli-zation of the source voltage is high.
If desired, the zero voltage firing circuit 90 may be formed of solid sta~e reIays.
~15 1 In the arrangements shown in Figures 52 and 53 the auxiliary source circuit including the step-up transformer ¦l is formed of components having stray capacitances between one another and with respect to ground ~ith the switch 72 I put in its open position. This results in a fear that a ¦ potential at the auxiliary electrodes 46 would be raised due to those stray capacitances until a voltage across the auxiliary electrode 46 and either of the main electrodes l and 2 exceeds the discharge breakdown voltage across the associated gap. Tllis results in the undesirable occurrence of a glow discharge between the main electrodes 1 and 2 which disables the principal glow discharge to be controlled with the pilot glow discharge.
In order to avoid this objection, the arrangement 9~

: I illustr ted in F;gure SS in~ludes a pnir of dummy resistors ~3 and 94 conneoted between the diode 73 and the resistor 71 ¦ and between the diode 74 and the resistor 71 respectively.
¦ ~he resistors 93 and 94 are e~fecti.ve for determining the ¦ potential at the auxiliary electrode 46 so as to prevent ¦ the voltage across the auxiliary electrodes 46 and either of ~:
¦ the main electrodes l and 2 from exceeding the discharge; r ¦ breakdown voltage across the gap as above described.
. ¦ In other respects, the arrangement is identical to ~;
l~ I that shown in Figure 53 except for the omission of the switch ::
,2.
¦ The auxi].iary electrode 46 is norlnally positioned ¦ to be equidistant from both main electrodes l and 2 and ¦ therefore the resistors 93 and 94 are equal in magnitude of :~
, lS ¦ resistance to each other in order to equal the voltage across ~ :
t.he auxiliary electrode 46 and the main electrode l to that ¦ across the electrodes 46 and 2 with the switch 62 put in its open position. Even under these circumstances, it is to ~:
.: ~e understood that the gap length between the auxiliary electrode 46 and either of the main electrodes l and 2, and the type and pressure of a dischargeable gas should be : preliminarily determined so as to prevent the occurrence of : a discharge b.reakdown between the auxiliary electrode 46 and :.
: either of the main electrodes l and 2 with the switch 62 put in its open position. -The arrangement illustrated in Figure 56 is different from that shown in Figure 55 only in that in Figure 56 a Triac is su~stituted for the switch 62 in order to permit ;~

.ll ~ 2~i the ON-OFF operation to be repeatedly performed with a high frequency. As shown in Figure 56, a Triac or a hidirectional triode thyristor 95 is located in place of the switch 62 shown in Figure 55. The Triac 95 includes a gate circuit 95 connected to a gate electrode thereof to deliver trigger signals to the gate electrode to turn the lriac 95 on and off and a series comhination of a capacitor 97 and a resistor 98 serving as an absorber.
If desired, the Triac 95 may be included in the zero ~roltage firing circuit 90.
When the pilot glow discharge has the discharge breakdown characteristic with a fairly long time delay, the pilot glow discharge may be fired at time point where the source voltage approaches its peak value provided that the Triac 95 has flowing therethrough a current an excess of its holding current. This is attended with the occurrence of the principal glow discharge having a sharply rising current A current for this glow discharge may sharply rise. In this case, a negative glow included in the principal discharge can not spread following an increase in current to locally concentrate the current resulting in a danger that the glow discharge transits to an arc discharge.
In order to avoid this danger, it is necessary to determine magnitudes of resistances 93 and 94 and an impedance on tlle primary side of the step-up transformer 70 enough to prevents a flow of current through the Triac 95 inexcess of its holding current.
In the arrangement illustrated in Figure 57 an `~:

. , ~. ,, ., .

,,.1..

- :~
`~ ~ :
electronic switch 98 such as a thyristor with a trigger circuit 99 is connected between the resistor 71 and the junction o~ dummy resistors 93 and 44 as shown in Figure 57.
When a voltage drop across the serially connected resistors 93 and 94 decrease to some extent, and when the electronic switch 98 is put in its ON state by the trigger circuit 99, a current flowing through the electronic switch 98 may exceed its holding current even in the absence of a pi.lot glow discharge. Under these circumstances, i the pilot glow discharge has the discharge brcakdown characteristic with a long time delay, ttlere is a danger that the resultillg glow discharge transits to an arc discharge as above described. In order to avoid this danger, the resistors 93 and 94 are required to high somewhat in resistance.
Alternatively the electronic switch 98 with its ::
trigger circuit 99 may be connected between the junction of the du~my resistors 93 and 94 and the auxiliary electrode 46 as shown in Figure 58. In these case, the resistors 93 and 94 are not particularly subjected to limitations as to .
their resistances unless a voltage across the auxiliary .
electrode 46 and either of the main electrodes 1 and 2 is reduced.
The arrangements shown in Figures 55 through 58 ::
ensure that the principal glow discharge is controlled with the pilot glow discharge. This is because, the dummy ~ :
resistors prevent the potential at the auxiliary electrode -from floating by mealls of stray capacitances as above described in conjunction with Figures 52 and 53 and the .

B`Zg5 ¦ like in the absence of the voltage applied to the auxiliary ¦ electrode.
The arrangement illustrated in Figure 59 comprises ~ :
: I an electrically isolating transformer 141 including a I primary winding connected across the AC source 31 and a secondary winding connected across a series combination of a rectifying dic.-le 142~ a current limiting resistor 143 ~ and capacitor 144, and an NPN transi.stor 149 including an ¦ emitter electrode connected to one side of the cal~acitor 144 and a collecto-r electrode connected to the other side of tlle capacitor 144 tllrol.lgh a sern;conductor di.ode 196 :Eor absorbing back pulses. The transistor 145 includes a base ¦~ e].ectrode connected to a gate circuit 149 also connected to I¦ the emitter electrode thereof to turn the transistor 145 on 1l and off.
The components 141 througll 146 form a high voltage pulse generator circuit generally designated by the reference numeral 140 wi.th a step-up pul.se transformer 147 which includes a primary winding connected across the diode 146 1 and a secondary winding connected to a semiconductor diode ;; . :
148 for shaping a pulse waveform.
: As in the arrangement of Fi~ure 57, the diode 148 is connected to the resistor 71 subse~uently connected to the auxiliary electrode 4h through the thyristor 98 which is turned on and off by a trigger circuit 99. Further the serially connected dummy resistors 93 and 94 are connected across the main electrodes 1 and 2 also through the switch 51 across the AC source with the junction of bcth resistors ~ 32~5 ~j . .' connected to the auxiliary electrode 46.
The opera-tion of the arrangement shown in Figure 59 ~ ~:
will now be described with reference to F:igure 60 wherein tllere are illustrated a voltage waveform V across the main electrodes 1 and 2 and a no-load voltage waveform VN at the auxiliary electrode 46. l~ith the main electrode 1 dlsposed oppositely to the main electrode 2 to -Eorm there- ~i between a predetermined gap fulfilling the relationship that tlle discharge breakdown voltagc V~ for the gap is :¦~
.lO h;gl~er than the ~ak value Em of the source vol.tage un~er . .
the predetermined discharge conditions the switch 51 is ¦I closed to apply the AC voltage across both electrodes 1 and from the source 31. Also the source 31 charges the capacitor Il 144 with the polarity illustrated through the transformer 141 ¦
I! the diode 142 and the resistor 143. Then gate and trigger ¦ circuits 149 and 99 respectively apply simultaneously ¦ respective gate signals to the trflnsistor 145 and the thyristor 99 to turn them on. The turn-on of the transistor 149 causes the charged capacitor 144 to discharge through the primary winding of the pulse transformer 147 and the now conducting transistor 145. As a result a pulse voltage stepped up by the pulse transformer 147 is supplied from the ~:
secondary winding thereof through the diode 148 the resistor 71 and the now conducting thyristor 98 to the auxiliary electrode 46. It is noted that the circuits 149 and 99 generate the respective pulses before the voltage across the maln electrode 1 and 2 reaches the discharge breakdown voltage VO. As shown in ~igure 60, the circuits 149 and ~' ~,.,',~Lr~ 5 99 generate the pulses at time point t2 before time point to w]lere the source voltage reaches the discharge brcakdown voltage VO in each positive half cycle thereof and the ~`
; pulses terminates short after time point to, That is, each pulse has a predetermined pulse width a little longer than a time interval between time points t2 and to. Each pulse i, is shown at waveform VN in Figure 60 as being superposed -' on that portion of the source voltage divided by the resistors 93 and 94, assuming that both resistors are equal in magnitucle of resistance to each other. In the next succeeding negative cycle of the source voltage the pulse is sirnilarly developed at time point t3 beEore time point tl where the voltage across the main electrodes l and 2 I .
reaches the negative value -VO of the discharge breakdown voltage and terminates short after time point tl to have the same pulse width as that appearing in the positive half-cycle of the source voltage. `;
In the arrangement of Figure 59 it is required to cause a pilot glow clischarge before time point to or tl by ,' applyillg the pulse waveform VN to the auxiliary electrode 46 as above described. Also it is required to select the pulse width so as to effect surely the discharge breakdown of the ~ap between the auxiliary electrode and either of ~-11 the main electrodes 1 and within the duration of the ¦1 associated pulse.
ll In general ? a time delay is caused after the voltage ¦ has been applied across discharge gaps and until the l I
I discharge breakdown is accomplished therebetween. It is well known that this time delay is equal to the sum of a time interval between the application of the voltage across dlscharge gap and the appearance of a first electron resulting in the initiation of development of the electron S avalanche and another time interval between the initiation of development of an electron avalanche and the completion of a stead-state disch~rge. The first mentional time interval is called a statistic delay and the la-tter is called a formatioTI delay, I`he statistic delay is over-po~eri]lgly long Assuming that a voltage applicd across the particular discharge gap has the peak value hig]ler that a voltage effecting the DC breakdown of the discharge gap, steped voltages are applied across the discharge gap _O times.
Assuming that, among them the _ applications of the voltage has time delays not shorter than T and (n + ~n) applications thereof has time delays not shorter than (T ~ ~T), ~n = -An~
holds where A designates a constant. Thus n nOe is fulfilled by the statistic delay. The above expression may be plotted into a straight line with the axes of ord;nates and abscissas representing the n and T respcctively in a semilogarithmic scale. A graphic representation thus , ,1 ' " `

~ ~:

plotted is called a Laue plot.
Figure 61 shows on example of the l,aue plot. In Figure 61 an extremity of an auxiliary electrode having a diameter of 3 millimeters is located at an edge of a gap of 3 millimeters formed between a pair of main opposite electrodes to forrn a spacing of about 1 millimeker betwee the extremity of the auxiliary electrode and either of the main electrodes. The gap was filled with a discharge gap formed of a mixture including 89% by volume of helium and ll% by volume of hydrogen under a pressure of l00 'rorrs.
[n Figure 61 the reEerence numerals 150, 151, 152 and 153 depict the source voltages having the peak values of 600, ~
800, 1000 and 1200 volts respectively. From a stepped ;~' ;
eurve 152, for example~ it is seen that for the peak source ~alue of 1000 volts the time interval between the t2 and to or between the t3 and tl (see Figure 60) must be of at least 250 microseconds. Also the auxiliary source for the pilot glow discharge should have a current capacity of at least about 10 milliamperes in order to transit smoothly the pilot glow discharge to the principal glow discharge.
By taking account of a tir,le delay with which the discharge gap is brown down with the pulse voltage of the voltage wavef-orm NN shown in Figure 60, the waveform ~N is given a pulse width or a duration defined by the time intervalc ranging from time point t2 or t3 to time point to or t respectively while the current capacity of the auxiliary source is determined as required for transiting the pilot glow discharge to the principal glow discharge and the z4~j;

pulse voltage delays rapidly at and after time point to or tl.
This measure ensures that the pilot glow discharge is always caused prior to time point to or tl and the principal discharge current surely rises at time point to or tl.
After the principal glow discharge has been caused ;~
between the main electrodes 1 and 2, discharge energy from the principal glow discharge as thermal energy alternately enters the main electrodes 1 and 2 with result tha-t a liquid flowing in contact relationship through either of the main electrodes is instantaneously heated.
; The arrangement of Figure 59 is advantageous in that the principal discharge current smooth}y rises to cause the devebpmentof a negative glow involved to satisfactorily follow up a change in discharge current thereby to prevent ~15 the local concentration of the cur~ent without the glow discharge transiting to an arc discharge while the efficiency of utilization of the source voltage. This is because the auxiliary electrode is adapte~ to be applied with a pulse voltage that rises before time pOillt where a voltage applied across the main electrodes reaches a glow hold minimum voltage across the main electrodes thereby to fire always the pilot glow discharge before that time point and Tapidly falls to its null value at and after said time point. Also the use of the pulse waveform is effective for decreasing the power capacity of the auxiliary source and therefore reducing a dimension and a cost thereof.
Figure 62 shows a modification of the arrangement shown in Figure 59. The arrangement illustrated comprises a 2~;

pair of electrically isolating transformers 141 and 155 :~
including a common iron core and a common primary winding :~
connected across the AC source 31 through the nor~ally open switch 51, the high voltage pulse generator circuit 14~ as ~ ~:
above described in conjunction with Figure 59 connected to ,`
~he transformer 141, and a current supply circuit generally cesignated by the refeTence numeral 154 and connected ' across the transformer 155. .
The current supply circuit 154 includes a center-t:apped secondary winding of the transformer 155, and a pair ,,~
of semiconductor diodes 156 and 157. The d:iode 156 is connected at the anode electrode to one side of the source 31 through the switch 51 and therefore the main electrode 1 while diode 157 is connected at the anode electrode to the lS other side of the source 31 and therefore the main electrode :~
2 that is, in turn, connected to ground. ~'he center tap on,the secondary transformer 155 winding is connected to the output of the pulse generator circuit 140 or the junction ;;~
of the diode 148 and the current limiting resistor 71~ ' In other respects, the arrangement is identical to that shown in Figure 59. The dot convention is used to ~ ;
identify the polarity of the instantaneous voltage developed across the associated transformer winding. ;~
The current supply circuit 155 is operative to full-wave rectify an AC voltage induced across the secondary transformer 155 winding and supply a current due to the full-wave rectified voltage to the auxiliary electrode 46 through the resistor 71 and the thyristor 9~ with the pulse - 100- ~
I ;' ,11 r .
voltage from the pulse generator circuit 140.
In the arrange~ent of Figure 62, the discharge gap ~.
between the main electrodes 1 and 2 has been dimensioned as above described in conjunc~ion with Figure 59 and the .
S switch 51 is closed to supply the source voltage across the .
: main electrodes 1 and 2. The source voltage is a commercial ~C voltage having a frequency of 60 hertzs as shown at dotted waveform V in Figure 63 wherein its cycle has a duration of 16.7 milliseconds.
The pulse generator circuit 140 generates a high : voltage pulse in each of the hal-f-cycles of the source voltage in the same manner as above described in conjunction with Figure 59. After having shaped by the diode 148, the high volts pulse is developed on the resistor 71 and superposed on the full-wave rectified voltage from the current supply circuit 154 also applied to the resistor 71 as shown at voltage waveform VN in Figure 62. Then pulse ~oltage VN superposed on the voltage from the current supply circuit 154 is supplied to the auxiary electrode 46 through the conducting thyristor 98.
From Figure 63 it is seen that the voltage waveform ~'N includes the full-wave rectified component having a relative voltage to the main electrode 2 equal to a voltage VOP for the pilot glow discharge at time point t6 in the positive half-cycle of the source voltage an also a relative voltage to the main electrode 1 equal to that voltage VOP
at time point t7 in the negative half-cycle thereof. Time . _ points ~6 and t7 are ahead of time points to and t ,ll ~ $;~

¦ respectively where the source voltage is equal to the glow ¦ hold minimum voltage VO.
¦ lYith the main electrode 1 higher in potential than ~ ¦ the main electrode 2, the diodes 156 is in its OFF state ; ¦ while the diode 157 is in its ON state tending to cause a ~;
pilot glow discharge between the auxiliary electrode 46 ¦ and the main electrode 2. On the contrary, with the main.
~; I electrode 1 less in potential than the main electrode 2, the ~ I diodes 156 and 157 are turned on and off respectively.
; 10 I This tends to cause a pilot glow discharge between the auxiliary electrode 46 and the main electrode 1. In each case, the voltage across the auxiliary and main electrode 16 and 1 respectively is equal to that across the auxiliary ¦ and main electrode 46 and 2 respectively so that a current ¦ for the pilot glow discharge remain unchanged. With the auxiliary electrode 46 equidistant from the main electrodes 1 and 2, the transit of the pilot glow discharge to the principal glow discharge between the main electrodes 1 and 2 is accomplished in the similar manner in both cases.
The voltage waveform VN also includes a pulse waveform component from the pulse generator circuit 140 rising at time point ~2 or t3 behind time point t6 or t2 ;
;~ and falling at time point t4 ahead of time point to or tl.
The pulse waveform component results from a gate pulse P
from either of the gate and trigger circuits 149 and 9 rising and falling simultaneously with the rise and fall of the associated pulse component. The pulse wave-form component is required to have a pulse width sufEicient to effect the .' - 1~2 , Il. .

discharge breakdown of the gap between the auxiliary electrode 46 and either of the main electrodes 1 and 2. It ~;
is to be noted that it is not required to cause time point ~4 or t5 to coincide ~ith time point t2 or tl respectively S as in the arrangement of Figure 59 and tha~ tlle pulse width may be sufficiently shorter than that reauired for the latter. In addition, the di.scharge breakdown scarcely requires a current resulting in the pulse generator circuit l .'40 reducing sufficiently in power capacity.
¦ The gate pulse from each of the gate and trigger circuits 149 and 99 should have a rise time fulfilling the ~ollowing re~uirements: The gate pulse Pl should rises at ~ime point t2 or t3 required to be behind time t6 or t7 ,.espectively while the pilot glow discharge should be caused not later than time point to or tl. Otherwise the principal discharge current i.s too sharply rai.sed to cause the spread I of the particular negative glow to follow this rise in current resulting in a danger that the current is locally ~:
concentrated on either of the main e].ectrode to permit the glow discharge to transit to an arc discharge. Also the source voltage can be utilized only l~ith a low ef-ficiency.
Thus time point t4 or t5 should be ahead of time point to or tl respectively.
With the gate pulse Pl generated to fulfill the requirements as above described, the pilot glow discharge is always caused ahead of time to or tl in the positive or negative half-cycle of the source voltage and the glow discharge current thro-lgh the mai~ electrodes 1 and 2 smoothly ,,~ .,ll I ~æ~
I ..
I . , ¦ rises at and after time point to or tl in the positive or :~
negative half-cycle of the source voltage. Accordingly, the principal glow discharge is established resulting in the l .nstantaneous heating of the particular liquid contacted by either of the main electrodes 1 and 2. ~ :
Further it is required to make the peak voltage value o the sinusoidal component of the voltage waveform ~/N less than the discharge breakdol~n voltage for the gap between the auxiliary electrode 46 and either of the main :- .
electrodes l and 2 thereby to prevent the pilot glow .~:~
discharge from firing with the sinusoi.dal component. -;
hlternatively, it is required to impart a high value to each of the resistance 93 or 94 to prevent the voltage waveform ~: ~
~N from being applied to the auxiliary electrode 46 in the :~ -lS ~ absence of the gate pulse Pl and to prevent a current flowing :-1:hrough the thyristor 98 via the reslstors 93 and 94 from :
exceeding the holding current thereo~ l~hen thc pilot glow discharge is not fired. Also the diodes 156 and 157 must : -~
have such reverse voltage withstandillg charac+er;stic that both diodes are not broken down with the high voltage plllses generated by the pulse generator circuit 14n.
If desired, the pulse generator circuit may utilize a peak transformer. ..
: The arrangement of Figure 62 is advantageous in :
that the pulse generator circuit can reduce in po~er .~ .
capacity resulting in the provision of an auxiliary soulce .
circuit easy to be manufactured and inexpensive. This is because the pulse generator circuit for ef.fecting the :

- 1~4 -ll ~ :~

discharge breakdown of the pilot glow discharge gap is separated from the circuit for supplying current to the main electrodes after this discharge breakdown.
Figure 64 shows a different modification of the present invention driven by a three-phase AC source. The arrangement illustrated comprises three rnain electrodes lU, lV and lW radially disposed by having their longitudinal axes arranged at equal angular intervals of 120 degrees.
The main electrodes are in the form of hollow cylinders having one end closed into a crown shape that, in turn, faces the remaining closed ends of ~he same shape. The main electrode lU, lV and lW include the other end portions rigidly itted ir-to respective annular supporting members 14U, 14V and 14W interconnected through enclosure portions 9 formed of an electrically insulating material such as glass po~celain or the like and seal fittings lOU9 lOV and lOW
connected to both adjacent suppo~rting members and the adjacent edges of the enclosure portions 9 to define a hermetic discharge space. The other ends of each electrode lU, lV or lW is closed with a blind cover plate 23U, 23V or 23W having an inflow tube 42U, 42V or 42W and an outflow tube 44U, 44V
or 44W is extended and sealed therethrough.
Three auxiliary electrodes 46U, 46V and 46W are radially extended and sealed throu~h the enclosure portions 9 respectively to be equidistant from the adjacent main electrodes and includes end portions bent toward the associatec main electrodes to form very narrow ~aps therebetween. For example, the auxiliary electrode 46U is radially extended and - l O S

,,1 ~.
~ . l~:

sealed through the enclosure portion 9 disposed between the main electrodes lU and lV and includes the end portion bent toward the main electrode lU so as to cause a pilot glow discharge. Each of the auxiliary electrodes is coated with the same electrically insulating material as the enclosure portion 9 except for both the end facing the associated -~
main electrode and that portion externally protruding from the mating cnclosure portion 9.
A three-phase source is represented by source terminals U, V and W which are connected to annlllar electrode terminals 6U, 6V and 6W fitted OlltO those portions of the main electrodes lU, lV and lW disposed externally of the enclosure portions 9 respectively. Each of the auxiliary electrodes is connected to the electrode terminals disposed on the adjacent main electrodes through individual dummy resistors. For example, the auxiliary electrode 46U is connected to the electrode terminal 6U of the main cylinder lU through the dummy resistor 47U on the one hand and to the electrode termirlal 6V of the main electrode lV through the dummy resistor 48U.
The auxiliary electrode 46U is also connected by a current limiting resistor 49U to an auxiliary source circuit 50 also connected to the electrode terminal 6U. The auxiliary source circuit 50 is further connected across the source terminals U and V through a normally open switch 51U connected to the source terminal V.
A circuit identical to that above described is provided -for each of the remaining main elec$rodes and the ~ ' ~ ~

l ~
I . :
l ~
¦ auxiliary electrode operatively associated therewith aJId ¦ includes the components identical to those above described.
¦ Therefore the identical components are clesignated by like ¦ reference numerals suffixed with the reference character ¦ U, V or W identifying the mating source terminal or the ¦ phase of the three-phase source.
¦ The operation of the arrangement shown in Figure 64 ¦ will now be described with reference to Figure 65 wherein ¦ there are illustrated voltage and current waveforms developed ¦ at various points in the arrangement of Figure 64 with a ¦ voltage Vu applied to the main ~lectrode lU being selected ~;
¦ as a reference.
While a liquid to be heated is flowing through the interior of each main electrode via the associated inflow tube and leaves the mating outflow tube a three phase voltage ¦ is applied to the main electrodes lU, lV and lW through the source terminals U, V and W and all the switches 51U, SlV
and 51W put in their closed position. At time point tl r short before a voltage (see waveform Vv, Figure 65) applied across the main electrodes lU and lV reaches a glow hold minimum voltage V0, a high voltage pulse (see waveform Puo, Figure 65) from the auxiliary source circuit S~U is applied to the auxiliary electrode 46U to cause a pilot glow -discharge across the narrow gap between the auxiliary electrode and main electrodes 46U and lU respectively with the main electrode lU acting as a cathode. This pilot glow discharge i5 caused with a low CIJrrent, and upont time point D being reached, it instantaneously induces a glow ~ ~ 4~

¦ discharge between the main electrodes lU and lV with the ¦ electrode lU acting as a cathode. The latter discharge ¦ spreads through the surface of both main electrodes lU and ¦ lV and is sustained after time point D.
5 ¦ Then when a voltage ~see waveform Vw, Figure 65) ¦ applied across the main electrodes lU and lW exceeds the ¦ glow hold minimum vol$age V0 at time point ~, the glow ¦ discharge developed be.ween the main electrodes lU and lV
¦ plays a role of the pilot glow discharge to cause a glow ¦ discharge between the main electrodes lU and lW at and ¦ after that time point with the main electrode lU acting as ¦ a cathode.
¦ At time point F volta~e across the main electrodes lV and lW is equal to the voltage V0 but no discharge is caused between those main electrodes because of the absence of a pilot glow discharge with the main electrode lV acting as a cathode. Therefore a high voltage pulse (see wave~orm PvO, Figure 65) from the auxiliary source circuit 50V is ;
applied to the auxiliary electrode 46V at time point t2 short ahead of time point F to cause a pilot glow discharge between the auxiliary and main e]ectrodes 46V and lV
respectively. That pllot glow discharge similarly causes a glow discharge between the main electrodes lV and lW at and after time point F with the main electrode lV acting as a cathode.
When time point G is reached, the voltage Vv across the main electrodes lU and lV is e~ual to the voltage V0 and the glow discharge caused between the main electrode lV

~1 acting as the cathode and the main electrode lW plays a role of a pilot glow discharge. This causes a glow discharge between the main electrode IV acting as a cathode and the main electrode lW at and after time point G.
Similarly, since the voltage Vw across the main electrodes lW and lU exceeds the voltage VO at time point H, a high voltage pulse (see waveform PwO, Figure 65) from the auxiliary source dircuit 50W has been preliminarily applied to the auxiliary electrode 46W at time point t3 short ~ lO ahead of time point H to cause a pilot glow discharge ; between the auxiliary electrode 46W and the main electrode acting as a cathode. The pilot glow discharge between the auxiliary and main electrode 46W and lW respectively trarlsits to a glow discharge caused between the main electrode lW
lS acting as a ca~hode and the main electrode lU at and after time point H. ~' Then at time ~oint I the voltage Vw across the main electrodes lV and lW exceeds the voltage VO so that the glow discharge between the main electrodes lW and lU ser~es as a pilot glow discharge to cause,a glow discharge between the main electrode lW acting as a cathode and the main electrode lU until one cycle of the source voltage is completed.
Thereafter the process as above described is repeated to cause repeatedly glow discharge between pairs of the main electrodes. When acting as the cathode, the main electrodes successively heat ~he liquid therein.
From the foregoing it will readily be understood - 1~9 -,, Il .

~ 2~5 ¦ that the gate pulses are repeatedly applied ~o the auxiliary ¦ electrodes 46U, 46V and 46W at time points t defined by ¦ t = tl + nT, t = t2 ~ nT and t = t3 + nT

¦ respectively where T designates a period of the three-phase ¦ source voltage and n indicate any positive integer including zerO.
¦ In Figure 65 solid current waveform IU designates ;~
1~ ¦ a glow discharge currents with the rnain electrode lU
¦ acting the cathode, dotted current ~aveform IV those with ¦ the main electrode iV acting as cathode and brokerl current ¦ waveform lW designates the glow discharge current with the ¦ main electrode lW acting as the cathode. The reference lS Uo~ PVo and P~O designate no-load pulse forms which or change to the actual pulse waveforms PU, PV
~` and PW respectively after the associated pilot glow ; discharges have been fired.
Also it is noted that Figure 65 illustrates the waveforms developed during a time interval equal to twice the period T of the source voltage Vv applied across the main electrodes lU and lV and that the polarity of the current waveforms have not been considered. ~
From the Eoregoing it will readily be understood ~ -~hat the glow discharge has a time period equal to three times that provided by single-phase system and therefore three-phase apparatus tripple in power capacity single-phase apparatus.

.. ,11 . ~
~ :~

In the arrangement of Figure 63 the auxiliary electrode is disposed between each pair of adjacent main electrodes for the purpose of controlling thermal energy entering each of the main electrodes. However it is included in the scope of the present invention to replace the auxiliary electrode by a bidirectional triode thyristor serially connected to each of the main electrode to control thermal energy entered thereinto tllrough the ON-OFF
operations of the thyristors.
The arrangement illustrated in Figure 66 is different from that shown in Figure 64 only in that in ;~
Figure 66 a co~bination of a pulse transformer 70U, 70V or 70W and a high voltage pulse generator circuit 140U, 140V
or 140W is substituted for each auxiliary source circuit.
The combination of the pulse transformer and pulse generator ;~
circuit may be identical to the pulse generator circuit 140 shown in Figure 59.
Also the main and auxiliary electrodes are schematically illustrated in Figure 66 and may be similar to those shown in Figure 64 and the resistors 48U, 48V and 48W are omitted. `~
Figure 67 shows another modification of the arrange-ment shown in Figure 66. In the arrangement illustrated, the electrically isolating transformer 70 includes a primary winding Wl connected across the source terminals U and V
through the switches 51 and a pair of secondary windings W2 and W3 connected respectively across a high voltage pulse generator circuit 140 such as above described in Z~S

conjunction with Figure 59 and a gate circuit 161, The pulse generator circuit 140 includes one output connected .:.
to the source terminal ~ and the other output connected to anode electrodes of thyristors Su, Sv and Sw through r the common curren~ limiting resistor 49. The thyristors Su, Sv and Sw include cathode electrodes connected to ~he . auxiliary electrodes 46U, 46V and 46W respectively. The : gate circuit 161 is connected to the thyristors Su, Sv and SW to control the firing thereo~.
In other respects, the arrangement is substantially identical to that shown in Figure 66.
Figure 69 illustrates voltage and current waveforms developed at various points in the arrangement shown in :~ Figure 67. From the comparison of Figure 68 with Figure 65 it is seen that voltage and current ~aveforms shown on the upper portion of Figure 68 are substantially similar to those illustrated in Figure 65 and pulse waveforms Po are substituted for the pulse waveforms PU-Puo, PV-Pvo and PW-Pwo shown in Figure 65. Thus like reference characters have been employed to identify the waveforms corresponding to those illustrated in Figure fi5. Thus the arrangement ~:
is substantially identical in operation to that shown in : Figure 66.
As seen in Figure 68, the ~ate circuit 161 applies a gate pulse (see waveform Gu) across the gate and cathode electrodes of the thyristor Su short before the high voltage pulse (see waveform Po from the pulse generator 140 is sup ied to the auxiliary electrode 46U to hring it in . 1 I

:
¦ its conducting state and then the pulse Po is supplied to ¦ the auxiliary electrode 46U through the resistor 49 and the ¦ now conducting thyristor Su. I`his is true in the case of ~
¦ the remaining pulses P passin~ through the respectlve ~ .
¦ thyristors Sv and Sw.
l Each of the gate pulses shown at waveforms Gu, GV
¦ and GW in Figure 68 should have a pulse width sufficient ;~
¦ to ensure that a pilot glow discharge is fired between the ¦ associated auxiliary and main electrodes such as shown by ;
¦ 46U and lU and transits to the principal glow discharge ¦ caused between the mating main electrodes such as shown by lU and lV. That is, the gate pulse should be at least sustained until time point is reached where the associated source voltage, for example, the voltage Vv exceeds the lS glow minimum voltage VO If the pilot glow discharge causes a current flowing through the associated thyristor to exceed i~s holding current then the gate pulse may continue until the pilot glow discharge is fired.
~ The arrangement of Figure 67 is advantageous over that shown in Figure 66 in that the resulting circuit is simple, small-sized and inexpensive because of the provision ~ `
of a single high voltage pulse generator circuit.
In the preferred embodiments of the present invention, the maln electrodes and associated components, such as the flow confining tubes, the connecting tubes, the inflow and ~ "~
outflow tube, the blind cover plates shown, for example, in Figure 24 are formed of metallic material and put in contact with a heated liquid that is electrolytic. This ., , .,.,., . ,, ,, . . . , . ~ ~

,,,.ll ~ 2~5 ~ ;;
,.
,., may result in a fear that those metallic components are corroded with the heated liquld and reduced in useful life.
Particularly the main electrodes and those tubes directly connected thereto have high probabilities of electrolytic corrosion because the source voltage is directly applied across the main electrodes while the inElow and outflow tubes are connected to grouned thereby to permit CuTrentS to flow in to the main electrodes and those tubes through the heated electrolytic liquid.
The arran~ement shown in ~igure 69 includes corrosion preventing electrodes for preventing metallic components from corroding as above described. In the arrangement illustrated a corrosion preventing electrode 161 or 162 is electrically insulatingly extended and sealed through that wall portion of the flow confining tube 20 or 21 facing the inside of the gap forming surface of the main electrode ~-2 or 1, that is, each of the opposite surfaces of both main electrodes with an electrically insulating holder 163 or 164 hermetically interposed therebetween. The electrode protrudes into the ~low path for the heated liquid. The anticorrosive electrode may be formed of platinum, carbon~
triiron tetroxide (~e304) or the like. The a DC source 165 or 166 is connected across the corrosion preventing electTode 161 or 162 and the electrode terminal 5 or 6 thereby to supply to the electrode 161 or 162 a voltage higher than the voltage across the main electrodes. To this end. Each of the DC source 165 or 166 includes a negative side connected to the associated electrode terminal 5 or 6.

Il ¢ . ! ~ . , ~
Then the electrode terminals 5 and 6 are connected .
to a control circuit identical to that shown in Figure 34~
: In other resyects, the arTangement is identical tothat shown in Figure 24 except for the prov;sion of the auxiliary electrode 46 but the main electrode 1, in this -:
case, made of stainless steel or the like, the flow confining tube 21, the blind cover plate 23, the connecti.ng tube 36, the insulating tubes 38 and 40, the inflow tube 42 ~-and the outflow tube 44 form an assembly prevented from corrodi.ng and generally designated by the reference numeral 167. Also, the similar components 2, 20, 22, 35, 37, 39, 41 and 43 form another assembly prevented from corroding and `.:
generally designated by the reference numeral 168. The ~:
main electrode 2 Is also made of stainless steel. ~`~
The corrosion of the main electrodes and others is called the electrolytic corrosion resulting from a flow of current therethrough via a heated electrolytic liquid that is caused from the dissolution of materials forming the main electrode and others into an electrolyte such as water. In the arrangement of Figure 69, the DC sources 165 and 166 are adapted to apply to the respective corrosion preventing electrodes 161 and 162 voltages higher that the voltage applied across the associated main electrodes. Thus the corrosion preventing electrodes 161 and 162 provide the so-called scapegoat electrodes. That is, the material or materials forming the scapegoat electrode is or are dissolved into an electrolyte such as water thereby to prevent the materials forming the assemblies 167 and 168 form dissolving Il ~ `:

into the heated liquid resulting in no corrosion occurring. `
The DC sources 165 and 166 may be omitted by forming the corrosion preventing electrode of a metallic material less in corrosion potential and more easily ionized than the material of the main electrode. For example, wi~h the .ain electrodes 1 and 2 formed of stainless steel, magnesium, zinc, aluminum, etc. are optimum for forming the corrosion ~reventing electrode.
Also the DC source may be repaced by any suitable source for supplying a DC voltage.
Figure 70 shows corrosion preventing electrodes Frovided on the arrangement shown in Figure 39. In Figure 70 the corrosion preventing electrodes 161 and 162 are provided on the exposed portion of the feed water tube 20 disposed within the main electrode 2 and on the outer wall of the main electrode 1 respectively in the same manner as above described in conjunction wit~ Figure 69.
Then the corrosion preventing electrodes 161 and 162 are connected to terminals d and e subsequently connected, for example, to the DC sources 165 and 166 (see Figure 69) respectively. Also terminals a and b connected to the electrode terminals 5 and 6 respectively are connected across the AC source 31 shown in Figure 69 while a terminal C connected to the auxiliary electrode 46 is connected to teh auxiliary source circuit 50 also shown in Figure 69.
Figure 71 shows anticorrosive electrodes provided on the arrangement illustrated in Figure 64. As shown, an anticorrosive electrode 161U, 161V or 161W is electrically ~ : ~

insulatingly extended and sealed tllrough the feed water tube 42U, 42V or 42W operatively coupled with each main electrode lU, lV or lW with an electrical.Ly insulating holder 164U, 164V or 174W interposed therebetween.
Figure 72 shows a separate modification of the present invention wilerein a temperature of a heated liquid is measured. In the arrangement illustrated, a temperature ,ensor 169 such as a thermistor is electrically insulatingly axtended and sealed through that portion of a flow confining ~ube 20 facing the peripheral wall of the main electrode 2 with an electrically insulating holder 170 interposed therebetween. ` ; .
The temperature sensor 169 may entirely covered with a electrically insulating material in accordance ~ ;~
lS with the particular electric field established in the vicinity ~hereof. , In other respects, the arrangement is substantially dentical to that shown in Figure 24 except for the provision of the auxiliary electrode 46.
The electrode terminals S and 6 and the auxiliary electrode 46 are connected to a control circuit identical to that shown in Figure 57 except -For the omission of the ¦ zero volt firing circuit 90. The temperature sensor 169 includes an output connected to the trigger circuit 99 for ¦ the thyristor 98.
In operation the temperature sensor 169 senses the temperature of the heated li~uid and feeds a measured temperature signal to the trigger circuit 99.

3zqS

~-l ~
More specifically, with the temperature sensor 169 formed . :~
of a thermistor or a temperature measuring resistor, a resistance thereof is changed with a temperature so that a :
cignal representative of a change in resistance is applied `
to the trigger circuit 99. Alternatively, Wit]l the tempera- :~
ture sensor 169 formed of a thermocouple, it responds to the temperature of the heated li~uid to change in thermo- :;
electromotive force thereof. This change in thermoelectro-motive force is signalled to the trigger circuit 99.
If it is desired to control the heated liquid to ; ~:
predetcrm;.ned fi.xed temperature then the actual tempera- ~:
t.ure measured by ~he temperature sensor 169 is compared with an output therefrom at a predetermined temperature as :.
the reference. With the actual temperature higher than :::
the predetermined temperature~ the trigger circuit g9 applies no trigger signal to the thyristor 98. Otherwise, :
the trigger circuit 99 delivers the trigger si.gnal to the ;
thyristor 98. Then the thyristor 9~ is correspondingly turned on and off to fire and extinguish a pilot glow dis- :;
charge thereby to effect the ON-OFF control of a glow discharge between the main electrodes 1 and 2.
: Under these circumstances, some time goes until heat from either of the main electrodes 1 and 2 acting as the heating surace is transferred to the heated li~uid.
This results in a time delay with ~hich the temperature of :
the heated liquid is controlled. Therefore, the temperature sensor 169 has preferably a sensor end located as near to ~ :
the heating sur~ace of the associated main electrode as ,.~. I . I
possible.
With the temperature of the heated temperature c.ontrolled according ~o a predetermi.ned program, the function of efEecting such control may be i.ncorporated into the ::
S trigger circuit 99 and the thyristor 98 is opera~ed in the .
ON-OFF control mode and in accordance with the output signal from the temperature sensor 169.
The temperature sensor 169 may be used with the control of the glow discharge effected by a control such as ~ ~ .
a thyristor connected in series to the particular glow discharge heating apparatus in a circuit wi.th an electri.c source circuit for the apparatus.
The arrangement illustrated in F:igure 73 is diE:Eerent ~`rom that shown in Figure 72 only in that in Figure 73 a bidirectional triode t~yristor or a Triac is provided to ¦
c.ontrol the glow discharge as in the arrangement of Figure 48. In Figure 73 a thyristor 172 is connected at the anode electrode to one of the DC output terminals of the :~
rectifier bridge 66 and at the cathode electrode to the :~
Triac 60. The resistor 67 is connected to the Triac 60 at the gate electrode but not to one main electrode thereof.
Then the th~ristor 172 has the cathode and gate electrodes connected across a trigger circuit 173 subse~uently connected to the temperature sensor 172.
In other respects, the control circuit is substan-ti.ally identical to that shown in Figure ~8. Ilowever the dot convention is used to identi:Ev the polarity of the instantaneous voltage across the assoc;ated transformer ~ 5 wlndlng .
The electrically isolating transformer 65 is opera-tive to adapt a potential difference deve:loped across the resi.stor 64 to a voltage required for the Triac 6a to be S fired.
With the switch 51 put in its closed position, the step-up transformer 70 applies a high AC voltage across t.he electrodes 1 and 2 resulting in the discharge breakdown cccurring therebetween. This cause a potential difference across the current limiting resistor 64 whereby a potential difference appears across the resistor 67 through the ~:
transformer 65 and the rectiEier bridge 66. At that time, the trigger circuit 173 is actuated to put the thyristor 172 in its ON state to cause a trigger signal to be applied to the gate electrode of the Triac 60 to turn it on. There-fore the AC source across the source 31 is supplied across ~1 the main electrodes 1 and 2 through the now conducting thyristor 6U to cause a glow discharge therebetween.
- Under these circumstances, the temperature sensor 169 senses a temperature of a heated liquid involved and feeds signal for the sensed temperature to the trigger -circuit to control the glow dischargc ~etween the :-electrodes 1 and 2.
:~ From the foregoing it is secn that in the arr~nge-ments shown in Figures 72 and 73 the temperature of the heated liquid sensed by the temperature sensor is fed to the auxiliary source circuit for controlling the glow dis charge caused between the main electrodes resulting in the ~ 2 ~ 3 ; easy, reliable temperature control of the heated liquid.
While the present invention has been illustrated and described in conjunction with various pre:Eerred embodi- : `;
ments thereof it is to be understood that numerous changes and modifications may be resorted to without departing from the spirit and scope of the present invention. For::
example, the embodiments of the present invention illustrat'ed and described in conjunction with the singlc-phase' source may readily be modified to be driven by the three-phase source. Sim.ilarly, the embodiments il.lustrated and descri.bed in conjunction with tlle three-phase source may readily be suit'ed for use with polyphase sources having m phases'~
where is greater than three ~3). In the latter case, an : m-phase AC voltages is applied to m main electrodes ~o cause successively glow discharges between the pairs thereof.~ :~
The resulting power capacity is equal to m times that ~:
provided by single-phase apparatus leading to inexpensive structures.
- ' ' ;`

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A glow discharge heating apparatus comprising a pair of discharge electrodes in the form of hollow cylinders having one end closed, said discharge electrodes being of the substantially same shape and disposed in opposite relationship by having said one closed ends opposing each other to form a predetermined gap therebetween, AC electric source means for applying an AC voltage across said discharge electrodes to cause a glow discharge therebetween, said glow discharge supplying thermal energy to the discharge electrode acting as a cathode during said glow discharge, and a liquid to be heated flowing through said discharge electrode acting as said cathode electrode to be heated with said thermal energy.

2. A glow discharge heating apparatus as claimed in claim 1, wherein said discharge electrodes are provided on portions thereof opposite to each other with concave and convex areas.

3. A glow discharge heating apparatus as claimed in claim 2, wherein said concave and convex area is arranged into a plurality of annuli, said annuli being in the form of concentric circles.

4. A glow discharge heating apparatus as claimed in claim 2, wherein said concave and convex area is in the form of straight lines running substantially in parallel relationship.

5. A glow discharge heating apparatus as claimed in claim 3, wherein said concave and convex area includes a plurality of depressions extending into said discharge electrode.